Australian Business Number

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An Australian Business Number, or ABN, is a unique identifier issued by the Australian Taxation Office (ATO) that is used by business entities when dealing with the Australian government. It was introduced as part of the New Tax System brought in by John Howard's Liberal government on 1 July 2000 as part of the Goods and Services Tax.

The main use of the ABN is on Tax Invoices. If the ABN is not disclosed on the tax invoice the purchasing business is not entitled to claim a GST credit on a purchase, and must withhold 46.5% of the sale price from the selling business to forward to the ATO.

[edit] Format of the ABN

The ABN is an eleven digit number where the first two digits are check digits. Unlike with the TFN, the ATO has publicised the formula for checking and creating valid ABN checksums. Also, the nature of the ABN algorithm means that any 9 digit number can be made into a valid ABN.

In the case of companies, the ATO determines the ABN by using the company's Australian Company Number (ACN) to which is prefixed the two-digit checksum.