diff --git a/CHANGELOG.md b/CHANGELOG.md
index e3319455c..27a7351d5 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,6 +6,10 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/) and this p
 
 ## [Unreleased]
 
+### Added
+
+- `au`: Added the Australian GST regime.
+
 ## [v0.309.0] - 2026-04-01
 
 ### Added
diff --git a/data/regimes/au.json b/data/regimes/au.json
new file mode 100644
index 000000000..1ba25325a
--- /dev/null
+++ b/data/regimes/au.json
@@ -0,0 +1,112 @@
+{
+  "$schema": "https://gobl.org/draft-0/tax/regime-def",
+  "name": {
+    "en": "Australia"
+  },
+  "description": {
+    "en": "Australia's indirect tax system is administered by the Australian\nTaxation Office (ATO). Goods and Services Tax (GST) applies at a\nstandard rate of 10%. Supplies may also be GST-free (for example, many\nexports) or input-taxed (for example, certain financial supplies and\nresidential rent). In GOBL's generic GST model, GST-free supplies map to\nthe zero key and input-taxed supplies map to the exempt key.\n\nBusinesses are identified by an Australian Business Number (ABN), an\n11-digit identifier used for GST registration and invoicing. Australian\ntax invoices must show the supplier's details and ABN, and invoices of\nAUD 1,000 or more, or self-billed invoices, must also identify the\ncustomer. Electronic invoicing is aligned with the Peppol PINT A-NZ\nspecification."
+  },
+  "sources": [
+    {
+      "title": {
+        "en": "ATO - GST"
+      },
+      "url": "https://www.ato.gov.au/businesses-and-organisations/gst-excise-and-indirect-taxes/gst"
+    },
+    {
+      "title": {
+        "en": "ATO - Tax invoices"
+      },
+      "url": "https://www.ato.gov.au/businesses-and-organisations/gst-excise-and-indirect-taxes/gst/tax-invoices"
+    },
+    {
+      "title": {
+        "en": "Peppol PINT A-NZ BIS"
+      },
+      "url": "https://docs.peppol.eu/poac/aunz/pint-aunz/bis/"
+    },
+    {
+      "title": {
+        "en": "ATO - eInvoicing"
+      },
+      "url": "https://www.ato.gov.au/businesses-and-organisations/invoicing-and-using-accounting-software/einvoicing"
+    }
+  ],
+  "time_zone": "Australia/Sydney",
+  "country": "AU",
+  "currency": "AUD",
+  "tax_scheme": "GST",
+  "corrections": [
+    {
+      "schema": "bill/invoice",
+      "types": [
+        "credit-note",
+        "debit-note"
+      ]
+    }
+  ],
+  "categories": [
+    {
+      "code": "GST",
+      "name": {
+        "en": "GST"
+      },
+      "title": {
+        "en": "Goods and Services Tax"
+      },
+      "keys": [
+        {
+          "key": "standard",
+          "name": {
+            "en": "Standard"
+          }
+        },
+        {
+          "key": "zero",
+          "name": {
+            "en": "Zero"
+          }
+        },
+        {
+          "key": "exempt",
+          "name": {
+            "en": "Exempt"
+          },
+          "no_percent": true
+        },
+        {
+          "key": "outside-scope",
+          "name": {
+            "en": "Outside scope"
+          },
+          "no_percent": true
+        }
+      ],
+      "rates": [
+        {
+          "rate": "general",
+          "keys": [
+            "standard"
+          ],
+          "name": {
+            "en": "General rate"
+          },
+          "values": [
+            {
+              "since": "2000-07-01",
+              "percent": "10%"
+            }
+          ]
+        }
+      ],
+      "sources": [
+        {
+          "title": {
+            "en": "ATO - GST"
+          },
+          "url": "https://www.ato.gov.au/businesses-and-organisations/gst-excise-and-indirect-taxes/gst"
+        }
+      ]
+    }
+  ]
+}
\ No newline at end of file
diff --git a/data/schemas/tax/regime-code.json b/data/schemas/tax/regime-code.json
index c4b6e938a..d0b7abb81 100644
--- a/data/schemas/tax/regime-code.json
+++ b/data/schemas/tax/regime-code.json
@@ -17,6 +17,10 @@
           "const": "AT",
           "title": "Austria"
         },
+        {
+          "const": "AU",
+          "title": "Australia"
+        },
         {
           "const": "BE",
           "title": "Belgium"
diff --git a/examples/au/invoice-au-au.yaml b/examples/au/invoice-au-au.yaml
new file mode 100644
index 000000000..b25f9c3d7
--- /dev/null
+++ b/examples/au/invoice-au-au.yaml
@@ -0,0 +1,41 @@
+$schema: https://gobl.org/draft-0/bill/invoice
+$regime: AU
+uuid: 2d301f3b-370a-4e15-a554-a8e05f61e93b
+currency: AUD
+series: "2026"
+code: "AU0001"
+issue_date: "2026-04-03"
+
+supplier:
+  name: Example Supplier Pty Ltd
+  tax_id:
+    country: AU
+    code: "51824753556"
+  addresses:
+    - street: George Street
+      num: "100"
+      locality: Sydney
+      region: NSW
+      code: "2000"
+      country: AU
+
+customer:
+  name: Example Customer Pty Ltd
+  addresses:
+    - street: Collins Street
+      num: "200"
+      locality: Melbourne
+      region: VIC
+      code: "3000"
+      country: AU
+
+lines:
+  - quantity: "1"
+    item:
+      name: Software engineering services
+      price: "900.00"
+      unit: h
+    taxes:
+      - cat: GST
+        rate: general
+        key: standard
diff --git a/examples/au/out/invoice-au-au.json b/examples/au/out/invoice-au-au.json
new file mode 100644
index 000000000..90fb5f843
--- /dev/null
+++ b/examples/au/out/invoice-au-au.json
@@ -0,0 +1,95 @@
+{
+	"$schema": "https://gobl.org/draft-0/envelope",
+	"head": {
+		"uuid": "8a51fd30-2a27-11ee-be56-0242ac120002",
+		"dig": {
+			"alg": "sha256",
+			"val": "9b83035fda7d9f82243097be6bfdfc321d431f25d4d355b2a00700ed5c9b6687"
+		}
+	},
+	"doc": {
+		"$schema": "https://gobl.org/draft-0/bill/invoice",
+		"$regime": "AU",
+		"uuid": "2d301f3b-370a-4e15-a554-a8e05f61e93b",
+		"type": "standard",
+		"series": "2026",
+		"code": "AU0001",
+		"issue_date": "2026-04-03",
+		"currency": "AUD",
+		"supplier": {
+			"name": "Example Supplier Pty Ltd",
+			"tax_id": {
+				"country": "AU",
+				"code": "51824753556"
+			},
+			"addresses": [
+				{
+					"num": "100",
+					"street": "George Street",
+					"locality": "Sydney",
+					"region": "NSW",
+					"code": "2000",
+					"country": "AU"
+				}
+			]
+		},
+		"customer": {
+			"name": "Example Customer Pty Ltd",
+			"addresses": [
+				{
+					"num": "200",
+					"street": "Collins Street",
+					"locality": "Melbourne",
+					"region": "VIC",
+					"code": "3000",
+					"country": "AU"
+				}
+			]
+		},
+		"lines": [
+			{
+				"i": 1,
+				"quantity": "1",
+				"item": {
+					"name": "Software engineering services",
+					"price": "900.00",
+					"unit": "h"
+				},
+				"sum": "900.00",
+				"taxes": [
+					{
+						"cat": "GST",
+						"key": "standard",
+						"rate": "general",
+						"percent": "10%"
+					}
+				],
+				"total": "900.00"
+			}
+		],
+		"totals": {
+			"sum": "900.00",
+			"total": "900.00",
+			"taxes": {
+				"categories": [
+					{
+						"code": "GST",
+						"rates": [
+							{
+								"key": "standard",
+								"base": "900.00",
+								"percent": "10%",
+								"amount": "90.00"
+							}
+						],
+						"amount": "90.00"
+					}
+				],
+				"sum": "90.00"
+			},
+			"tax": "90.00",
+			"total_with_tax": "990.00",
+			"payable": "990.00"
+		}
+	}
+}
diff --git a/regimes/au/README.md b/regimes/au/README.md
new file mode 100644
index 000000000..1b411ca32
--- /dev/null
+++ b/regimes/au/README.md
@@ -0,0 +1,47 @@
+# Australia (`AU`)
+
+Australia uses a Goods and Services Tax (GST) system administered by the Australian Taxation Office (ATO). GOBL models the Australian regime with a 10% standard GST rate, support for GST-free and input-taxed supplies through the generic GST key model, ABN validation, and invoice validation rules for supplier and customer identification.
+
+## Public Documentation
+
+- [ATO - GST](https://www.ato.gov.au/businesses-and-organisations/gst-excise-and-indirect-taxes/gst)
+- [ATO - Tax invoices](https://www.ato.gov.au/businesses-and-organisations/gst-excise-and-indirect-taxes/gst/tax-invoices)
+- [ABR - ABN format](https://abr.business.gov.au/Help/AbnFormat)
+- [Peppol PINT A-NZ BIS](https://docs.peppol.eu/poac/aunz/pint-aunz/bis/)
+
+## Tax Identity (ABN)
+
+Australian businesses are commonly identified by an Australian Business Number (ABN). The ABN is 11 digits long, usually written with spaces for display, but normalized in GOBL without separators.
+
+Validation follows the ABR checksum algorithm:
+
+| Position | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 |
+| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
+| Weight | 10 | 1 | 3 | 5 | 7 | 9 | 11 | 13 | 15 | 17 | 19 |
+
+1. Subtract 1 from the first digit.
+2. Multiply each digit by its weight.
+3. Sum the products.
+4. The total must be divisible by 89.
+
+## GST
+
+Australian GST distinguishes between taxable supplies, GST-free supplies, and input-taxed supplies. GOBL keeps Australia on the shared GST model and maps those concepts as follows:
+
+| Australian concept | GOBL key / handling | GST treatment |
+| --- | --- | --- |
+| Taxable supply | `standard` / `general` | 10% GST |
+| GST-free supply | `zero` | 0% GST through the shared GST zero key |
+| Input-taxed supply | `exempt` | No GST charged; used as the generic mapping for input-taxed treatment |
+| Outside scope / non-taxable | `outside-scope` | Not part of the GST calculation |
+
+| Rate Name | GOBL Rate Key | Percent | Since |
+| --- | --- | --- | --- |
+| General rate | `standard` / `general` | 10% | 2000-07-01 |
+
+## Tax Invoices
+
+- Suppliers must include their details and ABN.
+- Invoices of AUD 1,000 or more must identify the customer.
+- Self-billed invoices must identify the customer regardless of amount.
+- In this implementation pass, customer identification is satisfied by the customer's name; an AU ABN may also be included but is not required when the name is present.
diff --git a/regimes/au/au.go b/regimes/au/au.go
new file mode 100644
index 000000000..b6b53bb77
--- /dev/null
+++ b/regimes/au/au.go
@@ -0,0 +1,95 @@
+// Package au provides models for dealing with Australia.
+package au
+
+import (
+	"github.com/invopop/gobl/bill"
+	"github.com/invopop/gobl/cbc"
+	"github.com/invopop/gobl/currency"
+	"github.com/invopop/gobl/i18n"
+	"github.com/invopop/gobl/l10n"
+	"github.com/invopop/gobl/pkg/here"
+	"github.com/invopop/gobl/tax"
+)
+
+func init() {
+	tax.RegisterRegimeDef(New())
+}
+
+// New instantiates a new Australian regime.
+func New() *tax.RegimeDef {
+	return &tax.RegimeDef{
+		Country:   l10n.AU.Tax(),
+		Currency:  currency.AUD,
+		TaxScheme: tax.CategoryGST,
+		Name: i18n.String{
+			i18n.EN: "Australia",
+		},
+		Description: i18n.String{
+			i18n.EN: here.Doc(`
+					Australia's indirect tax system is administered by the Australian
+					Taxation Office (ATO). Goods and Services Tax (GST) applies at a
+					standard rate of 10%. Supplies may also be GST-free (for example, many
+					exports) or input-taxed (for example, certain financial supplies and
+					residential rent). In GOBL's generic GST model, GST-free supplies map to
+					the zero key and input-taxed supplies map to the exempt key.
+
+					Businesses are identified by an Australian Business Number (ABN), an
+					11-digit identifier used for GST registration and invoicing. Australian
+					tax invoices must show the supplier's details and ABN, and invoices of
+					AUD 1,000 or more, or self-billed invoices, must also identify the
+					customer. Electronic invoicing is aligned with the Peppol PINT A-NZ
+					specification.
+				`),
+		},
+		Sources: []*cbc.Source{
+			{
+				Title: i18n.NewString("ATO - GST"),
+				URL:   "https://www.ato.gov.au/businesses-and-organisations/gst-excise-and-indirect-taxes/gst",
+			},
+			{
+				Title: i18n.NewString("ATO - Tax invoices"),
+				URL:   "https://www.ato.gov.au/businesses-and-organisations/gst-excise-and-indirect-taxes/gst/tax-invoices",
+			},
+			{
+				Title: i18n.NewString("Peppol PINT A-NZ BIS"),
+				URL:   "https://docs.peppol.eu/poac/aunz/pint-aunz/bis/",
+			},
+			{
+				Title: i18n.NewString("ATO - eInvoicing"),
+				URL:   "https://www.ato.gov.au/businesses-and-organisations/invoicing-and-using-accounting-software/einvoicing",
+			},
+		},
+		TimeZone: "Australia/Sydney",
+		Corrections: []*tax.CorrectionDefinition{
+			{
+				Schema: bill.ShortSchemaInvoice,
+				Types: []cbc.Key{
+					bill.InvoiceTypeCreditNote,
+					bill.InvoiceTypeDebitNote,
+				},
+			},
+		},
+		Validator:  Validate,
+		Normalizer: Normalize,
+		Categories: taxCategories(),
+	}
+}
+
+// Validate checks the document type and determines if it can be validated.
+func Validate(doc any) error {
+	switch obj := doc.(type) {
+	case *bill.Invoice:
+		return validateBillInvoice(obj)
+	case *tax.Identity:
+		return validateTaxIdentity(obj)
+	}
+	return nil
+}
+
+// Normalize will attempt to clean the object passed to it.
+func Normalize(doc any) {
+	switch obj := doc.(type) {
+	case *tax.Identity:
+		tax.NormalizeIdentity(obj)
+	}
+}
diff --git a/regimes/au/au_test.go b/regimes/au/au_test.go
new file mode 100644
index 000000000..f12f3755c
--- /dev/null
+++ b/regimes/au/au_test.go
@@ -0,0 +1,65 @@
+package au_test
+
+import (
+	"encoding/json"
+	"strings"
+	"testing"
+
+	"github.com/invopop/gobl/bill"
+	"github.com/invopop/gobl/currency"
+	"github.com/invopop/gobl/l10n"
+	"github.com/invopop/gobl/regimes/au"
+	"github.com/invopop/gobl/tax"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestNew(t *testing.T) {
+	t.Parallel()
+
+	regime := au.New()
+
+	assert.Equal(t, l10n.AU.Tax(), regime.Country)
+	assert.Equal(t, currency.AUD, regime.Currency)
+	assert.Equal(t, tax.CategoryGST, regime.TaxScheme)
+	assert.Equal(t, "Australia", regime.Name.String())
+	assert.NotEmpty(t, regime.Categories)
+	assert.NotNil(t, regime.Validator)
+	assert.NotNil(t, regime.Normalizer)
+	assert.Len(t, regime.Corrections, 1)
+	assert.Equal(t, []string{
+		bill.InvoiceTypeCreditNote.String(),
+		bill.InvoiceTypeDebitNote.String(),
+	}, []string{
+		regime.Corrections[0].Types[0].String(),
+		regime.Corrections[0].Types[1].String(),
+	})
+	assert.Contains(t, regime.Description.String(), "GST-free")
+	assert.Contains(t, regime.Description.String(), "input-taxed")
+	assert.True(t, strings.Contains(regime.Description.String(), "exempt key"))
+	assert.NotNil(t, regime.CategoryDef(tax.CategoryGST))
+	assert.NotNil(t, regime.CategoryDef(tax.CategoryGST).KeyDef(tax.KeyExempt))
+}
+
+func TestRegimeValidation(t *testing.T) {
+	t.Parallel()
+
+	regime := au.New()
+	require.NoError(t, regime.Validate())
+}
+
+func TestCorrectionOptionsSchema(t *testing.T) {
+	t.Parallel()
+
+	inv := validInvoice()
+	require.NoError(t, inv.Calculate())
+
+	out, err := inv.CorrectionOptionsSchema()
+	require.NoError(t, err)
+
+	data, err := json.Marshal(out)
+	require.NoError(t, err)
+
+	assert.Contains(t, string(data), `"const":"credit-note"`)
+	assert.Contains(t, string(data), `"const":"debit-note"`)
+}
diff --git a/regimes/au/bill_invoice.go b/regimes/au/bill_invoice.go
new file mode 100644
index 000000000..87ef40adc
--- /dev/null
+++ b/regimes/au/bill_invoice.go
@@ -0,0 +1,86 @@
+package au
+
+import (
+	"github.com/invopop/gobl/bill"
+	"github.com/invopop/gobl/l10n"
+	"github.com/invopop/gobl/num"
+	"github.com/invopop/gobl/org"
+	"github.com/invopop/gobl/tax"
+	"github.com/invopop/validation"
+)
+
+var customerIdentificationThreshold = num.MakeAmount(100000, 2)
+
+func validateBillInvoice(inv *bill.Invoice) error {
+	if inv == nil {
+		return nil
+	}
+	return validation.ValidateStruct(inv,
+		validation.Field(&inv.Supplier,
+			validation.By(validateBillInvoiceSupplier),
+			validation.Skip,
+		),
+		validation.Field(&inv.Customer,
+			validation.When(
+				requiresCustomerIdentification(inv),
+				validation.Required,
+			).Else(
+				validation.Skip,
+			),
+			validation.By(validateBillInvoiceCustomer(inv)),
+			validation.Skip,
+		),
+	)
+}
+
+func validateBillInvoiceSupplier(value any) error {
+	party, ok := value.(*org.Party)
+	if !ok || party == nil {
+		return nil
+	}
+	return validation.ValidateStruct(party,
+		validation.Field(&party.Name,
+			validation.Required,
+			validation.Skip,
+		),
+		validation.Field(&party.TaxID,
+			validation.Required,
+			tax.RequireIdentityCode,
+			validation.By(validateBillInvoiceSupplierTaxID),
+			validation.Skip,
+		),
+	)
+}
+
+func validateBillInvoiceSupplierTaxID(value any) error {
+	tID := value.(*tax.Identity)
+	return validation.ValidateStruct(tID,
+		validation.Field(&tID.Country,
+			validation.In(l10n.AU.Tax()),
+			validation.Skip,
+		),
+	)
+}
+
+func validateBillInvoiceCustomer(inv *bill.Invoice) validation.RuleFunc {
+	return func(value any) error {
+		party, ok := value.(*org.Party)
+		if !ok || party == nil || !requiresCustomerIdentification(inv) {
+			return nil
+		}
+		return validation.ValidateStruct(party,
+			validation.Field(&party.Name,
+				validation.Required,
+				validation.Skip,
+			),
+		)
+	}
+}
+
+func requiresCustomerIdentification(inv *bill.Invoice) bool {
+	if inv.HasTags(tax.TagSelfBilled) {
+		return true
+	}
+	return inv.Totals != nil &&
+		inv.Totals.TotalWithTax.Compare(customerIdentificationThreshold) >= 0
+}
diff --git a/regimes/au/bill_invoice_test.go b/regimes/au/bill_invoice_test.go
new file mode 100644
index 000000000..91d2c13da
--- /dev/null
+++ b/regimes/au/bill_invoice_test.go
@@ -0,0 +1,177 @@
+package au_test
+
+import (
+	"testing"
+	"time"
+
+	"github.com/invopop/gobl/bill"
+	"github.com/invopop/gobl/cal"
+	"github.com/invopop/gobl/currency"
+	"github.com/invopop/gobl/l10n"
+	"github.com/invopop/gobl/num"
+	"github.com/invopop/gobl/org"
+	"github.com/invopop/gobl/regimes/au"
+	"github.com/invopop/gobl/tax"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func validInvoice() *bill.Invoice {
+	return &bill.Invoice{
+		Regime:    tax.WithRegime("AU"),
+		Series:    "2026",
+		Code:      "AU0001",
+		IssueDate: cal.MakeDate(2026, time.April, 3),
+		Currency:  currency.AUD,
+		Supplier: &org.Party{
+			Name: "Example Supplier Pty Ltd",
+			TaxID: &tax.Identity{
+				Country: l10n.AU.Tax(),
+				Code:    "51824753556",
+			},
+			Addresses: []*org.Address{
+				{
+					Street:   "George Street",
+					Number:   "100",
+					Locality: "Sydney",
+					State:    "NSW",
+					Code:     "2000",
+					Country:  l10n.AU.ISO(),
+				},
+			},
+		},
+		Customer: &org.Party{
+			Name: "Example Customer Pty Ltd",
+			Addresses: []*org.Address{
+				{
+					Street:   "Collins Street",
+					Number:   "200",
+					Locality: "Melbourne",
+					State:    "VIC",
+					Code:     "3000",
+					Country:  l10n.AU.ISO(),
+				},
+			},
+		},
+		Lines: []*bill.Line{
+			{
+				Quantity: num.MakeAmount(1, 0),
+				Item: &org.Item{
+					Name:  "Software engineering services",
+					Price: num.NewAmount(90000, 2),
+					Unit:  org.UnitHour,
+				},
+				Taxes: tax.Set{
+					{
+						Category: tax.CategoryGST,
+						Rate:     tax.RateGeneral,
+					},
+				},
+			},
+		},
+	}
+}
+
+func TestInvoiceValidation(t *testing.T) {
+	t.Parallel()
+
+	t.Run("nil invoice", func(t *testing.T) {
+		t.Parallel()
+		var inv *bill.Invoice
+		require.NoError(t, au.Validate(inv))
+	})
+
+	t.Run("valid invoice under threshold", func(t *testing.T) {
+		t.Parallel()
+		inv := validInvoice()
+		require.NoError(t, inv.Calculate())
+		require.NoError(t, inv.Validate())
+	})
+
+	t.Run("valid invoice at threshold with customer name only", func(t *testing.T) {
+		t.Parallel()
+		inv := validInvoice()
+		inv.Lines[0].Item.Price = num.NewAmount(100000, 2)
+		require.NoError(t, inv.Calculate())
+		require.NoError(t, inv.Validate())
+	})
+
+	t.Run("invoice at threshold without customer", func(t *testing.T) {
+		t.Parallel()
+		inv := validInvoice()
+		inv.Lines[0].Item.Price = num.NewAmount(100000, 2)
+		inv.Customer = nil
+		require.NoError(t, inv.Calculate())
+		err := inv.Validate()
+		assert.ErrorContains(t, err, "customer")
+	})
+
+	t.Run("self billed invoice under threshold with customer name", func(t *testing.T) {
+		t.Parallel()
+		inv := validInvoice()
+		inv.SetTags(tax.TagSelfBilled)
+		require.NoError(t, inv.Calculate())
+		require.NoError(t, inv.Validate())
+	})
+
+	t.Run("self billed invoice under threshold without customer", func(t *testing.T) {
+		t.Parallel()
+		inv := validInvoice()
+		inv.SetTags(tax.TagSelfBilled)
+		inv.Customer = nil
+		require.NoError(t, inv.Calculate())
+		err := inv.Validate()
+		assert.ErrorContains(t, err, "customer")
+	})
+
+	t.Run("regime validator with nil supplier party", func(t *testing.T) {
+		t.Parallel()
+		inv := validInvoice()
+		inv.Supplier = nil
+		require.NoError(t, au.Validate(inv))
+	})
+
+	t.Run("nil supplier", func(t *testing.T) {
+		t.Parallel()
+		inv := validInvoice()
+		inv.Supplier = nil
+		require.NoError(t, inv.Calculate())
+		require.Error(t, inv.Validate())
+	})
+
+	t.Run("missing supplier tax ID", func(t *testing.T) {
+		t.Parallel()
+		inv := validInvoice()
+		inv.Supplier.TaxID = nil
+		require.NoError(t, inv.Calculate())
+		err := inv.Validate()
+		assert.ErrorContains(t, err, "tax_id")
+	})
+
+	t.Run("missing supplier name", func(t *testing.T) {
+		t.Parallel()
+		inv := validInvoice()
+		inv.Supplier.Name = ""
+		require.NoError(t, inv.Calculate())
+		err := inv.Validate()
+		assert.ErrorContains(t, err, "name")
+	})
+
+	t.Run("supplier tax ID must be australian", func(t *testing.T) {
+		t.Parallel()
+		inv := validInvoice()
+		inv.Supplier.TaxID.Country = "US"
+		require.NoError(t, inv.Calculate())
+		err := inv.Validate()
+		assert.ErrorContains(t, err, "country")
+	})
+
+	t.Run("supplier tax ID must be valid ABN", func(t *testing.T) {
+		t.Parallel()
+		inv := validInvoice()
+		inv.Supplier.TaxID.Code = "11111111111"
+		require.NoError(t, inv.Calculate())
+		err := inv.Validate()
+		assert.ErrorContains(t, err, "invalid checksum")
+	})
+}
diff --git a/regimes/au/tax_categories.go b/regimes/au/tax_categories.go
new file mode 100644
index 000000000..5d14b2cbf
--- /dev/null
+++ b/regimes/au/tax_categories.go
@@ -0,0 +1,46 @@
+package au
+
+import (
+	"github.com/invopop/gobl/cal"
+	"github.com/invopop/gobl/cbc"
+	"github.com/invopop/gobl/i18n"
+	"github.com/invopop/gobl/num"
+	"github.com/invopop/gobl/tax"
+)
+
+func taxCategories() []*tax.CategoryDef {
+	return []*tax.CategoryDef{
+		{
+			Code: tax.CategoryGST,
+			Name: i18n.String{
+				i18n.EN: "GST",
+			},
+			Title: i18n.String{
+				i18n.EN: "Goods and Services Tax",
+			},
+			Sources: []*cbc.Source{
+				{
+					Title: i18n.NewString("ATO - GST"),
+					URL:   "https://www.ato.gov.au/businesses-and-organisations/gst-excise-and-indirect-taxes/gst",
+				},
+			},
+			Retained: false,
+			Keys:     tax.GlobalGSTKeys(),
+			Rates: []*tax.RateDef{
+				{
+					Keys: []cbc.Key{tax.KeyStandard},
+					Rate: tax.RateGeneral,
+					Name: i18n.String{
+						i18n.EN: "General rate",
+					},
+					Values: []*tax.RateValueDef{
+						{
+							Since:   cal.NewDate(2000, 7, 1),
+							Percent: num.MakePercentage(10, 2),
+						},
+					},
+				},
+			},
+		},
+	}
+}
diff --git a/regimes/au/tax_identity.go b/regimes/au/tax_identity.go
new file mode 100644
index 000000000..ab1aa94db
--- /dev/null
+++ b/regimes/au/tax_identity.go
@@ -0,0 +1,57 @@
+package au
+
+import (
+	"errors"
+	"strconv"
+	"strings"
+
+	"github.com/invopop/gobl/cbc"
+	"github.com/invopop/gobl/tax"
+	"github.com/invopop/validation"
+)
+
+const abnLength = 11
+
+var abnWeights = []int{10, 1, 3, 5, 7, 9, 11, 13, 15, 17, 19}
+
+// validateTaxIdentity performs validation specific to Australian tax IDs.
+func validateTaxIdentity(tID *tax.Identity) error {
+	if tID == nil {
+		return nil
+	}
+	return validation.ValidateStruct(tID,
+		validation.Field(&tID.Code,
+			validation.By(validateABN),
+			validation.Skip,
+		),
+	)
+}
+
+// validateABN checks Australian Business Numbers (ABNs).
+// Reference: https://abr.business.gov.au/Help/AbnFormat
+func validateABN(value any) error {
+	code, _ := value.(cbc.Code)
+	normalized := strings.ReplaceAll(strings.ToUpper(code.String()), " ", "")
+	if normalized == "" {
+		return nil
+	}
+	if len(normalized) != abnLength {
+		return errors.New("invalid length")
+	}
+	if _, err := strconv.Atoi(normalized); err != nil {
+		return errors.New("invalid characters, expected numeric")
+	}
+
+	sum := 0
+	for i, r := range normalized {
+		digit := int(r - '0')
+		if i == 0 {
+			digit--
+		}
+		sum += digit * abnWeights[i]
+	}
+	if sum%89 != 0 {
+		return errors.New("invalid checksum")
+	}
+	return nil
+}
diff --git a/regimes/au/tax_identity_test.go b/regimes/au/tax_identity_test.go
new file mode 100644
index 000000000..0cf942da1
--- /dev/null
+++ b/regimes/au/tax_identity_test.go
@@ -0,0 +1,82 @@
+package au_test
+
+import (
+	"testing"
+
+	"github.com/invopop/gobl/cbc"
+	"github.com/invopop/gobl/regimes/au"
+	"github.com/invopop/gobl/tax"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestValidateTaxIdentity(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name        string
+		code        cbc.Code
+		expectedErr string
+	}{
+		{name: "valid ABN from ATO", code: "51824753556"},
+		{name: "valid ABN with spaces", code: "51 824 753 556"},
+		{name: "second valid ABN", code: "53004085616"},
+		{name: "empty ABN", code: ""},
+		{name: "too short", code: "1234567890", expectedErr: "invalid length"},
+		{name: "too long", code: "123456789012", expectedErr: "invalid length"},
+		{name: "invalid checksum", code: "11111111111", expectedErr: "invalid checksum"},
+		{name: "contains non-numeric characters", code: "5182475355A", expectedErr: "invalid characters, expected numeric"},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+			tID := &tax.Identity{Country: "AU", Code: tt.code}
+
+			err := au.Validate(tID)
+
+			if tt.expectedErr == "" {
+				assert.NoError(t, err)
+			} else if assert.Error(t, err) {
+				assert.Contains(t, err.Error(), tt.expectedErr)
+			}
+		})
+	}
+
+	t.Run("nil identity", func(t *testing.T) {
+		var tID *tax.Identity
+		assert.NoError(t, au.Validate(tID))
+	})
+}
+
+func TestNormalizeTaxIdentity(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name     string
+		input    cbc.Code
+		expected cbc.Code
+	}{
+		{name: "spaces stripped", input: "51 824 753 556", expected: "51824753556"},
+		{name: "country prefix stripped", input: "AU51824753556", expected: "51824753556"},
+		{name: "already normalized", input: "51824753556", expected: "51824753556"},
+		{name: "empty code", input: "", expected: ""},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+			tID := &tax.Identity{Country: "AU", Code: tt.input}
+
+			au.Normalize(tID)
+
+			assert.Equal(t, tt.expected, tID.Code)
+		})
+	}
+
+	t.Run("nil identity", func(t *testing.T) {
+		assert.NotPanics(t, func() {
+			var tID *tax.Identity
+			au.Normalize(tID)
+		})
+	})
+}
diff --git a/regimes/regimes.go b/regimes/regimes.go
index ff3cb73a8..a9a774813 100644
--- a/regimes/regimes.go
+++ b/regimes/regimes.go
@@ -8,6 +8,7 @@ import (
 	_ "github.com/invopop/gobl/regimes/ae"
 	_ "github.com/invopop/gobl/regimes/ar"
 	_ "github.com/invopop/gobl/regimes/at"
+	_ "github.com/invopop/gobl/regimes/au"
 	_ "github.com/invopop/gobl/regimes/be"
 	_ "github.com/invopop/gobl/regimes/br"
 	_ "github.com/invopop/gobl/regimes/ca"