Australian GST Validation API: ABN Format, ABR Lookup, and GST Registration

Australian GST validation is based on the Australian Business Number (ABN), an 11-digit identifier issued by the Australian Business Register (ABR). This guide covers how ABNs and GST registration work, and how Vatly validates them for you.

What is an ABN?

An ABN (Australian Business Number) is an 11-digit unique identifier for businesses operating in Australia. It is issued by the Australian Business Register and is required for tax and business transactions. In Vatly, Australian ABNs use the AU country prefix. Example: AU51824753556.

• AU - country prefix (Vatly convention)
• 11 digits - the ABN itself (first two are check digits)

You can also pass the ABN prefix (e.g., ABN51824753556) and Vatly normalizes it to AU51824753556 automatically.

ABN vs GST registration

Having an ABN does not automatically mean the business is registered for GST (Goods and Services Tax). Businesses must register for GST separately once their annual turnover reaches AUD 75,000 (AUD 150,000 for non-profit organizations).

Vatly checks both: valid: true means the ABN exists and the business has an active GST registration. If the ABN exists but has no GST registration, you get valid: false with the company name and address still included.

The ABR Lookup service

The Australian Business Register provides a public API for looking up ABN details. Key characteristics:

• Protocol: JSONP (callback wrapper that needs to be stripped)
• Authentication: requires a GUID (API key) obtained by registration
• Returns: business name, ABN status, GST registration date, address (state + postcode)
• Free to use

The ABR API returns GST registration as a date field. If the field contains a date, the business is currently GST-registered. If it is empty, the business is not GST-registered.

Why wrapping the ABR matters

The ABR API has several characteristics that make direct integration painful:

• JSONP response format requires stripping the callback wrapper before parsing
• No structured error responses for invalid ABNs
• No built-in caching or rate limiting
• Authentication requires registering for a GUID

Vatly wraps the ABR in the same REST API, JSON format, caching layer, and error handling you already use for EU, UK, and other validations. No additional integration work required.

ABN checksum validation

ABNs include a checksum for detecting data entry errors. The algorithm works differently from MOD11 used by Swiss and Norwegian numbers:

1. Subtract 1 from the first digit
2. Multiply each of the 11 digits by the weights [10, 1, 3, 5, 7, 9, 11, 13, 15, 17, 19]
3. Sum all products
4. The ABN is valid if the sum is divisible by 89

Vatly validates this checksum before calling the ABR upstream. Invalid checksums return a 422 invalid_vat_format error immediately, saving you an unnecessary API call.

Validating with Vatly

Pass any Australian ABN to the same endpoint you use for EU, UK, Swiss, and Norwegian numbers:

curl "https://api.vatly.dev/v1/validate?vat_number=AU51824753556" \
  -H "Authorization: Bearer vtly_live_YOUR_KEY"

Response:

{
  "data": {
    "valid": true,
    "vat_number": "AU51824753556",
    "country_code": "AU",
    "company": {
      "name": "COMMONWEALTH BANK OF AUSTRALIA",
      "address": "NSW 2000"
    },
    "requested_at": "2026-03-26T12:00:00.000Z"
  },
  "meta": {
    "request_id": "req_abc123"
  }
}

Vatly normalizes all input formats:

• AU51824753556 and ABN51824753556 both work
• The ABN prefix is converted to AU automatically
• Checksum is validated before making the upstream call
• Results are cached for 25 days

Australian GST rates

Australia has a flat 10% GST rate with no reduced rates. GST-free items (fresh food, health, education) are zero-rated. These are available through the /v1/rates endpoint.

Test mode

Use a vtly_test_ API key with any valid ABN checksum. For example:

• AU51824753556 - returns valid with test company data

Any ABN that passes the checksum validation will return a test response in test mode. Test mode never hits the ABR, so your integration tests are fast and deterministic.

Get your API key and start validating Australian ABNs in under 2 minutes.