An Australian Business Number (ABN) is a unique 11 digit number that identifies your business to the government and community. An ABN doesn't replace your tax file number, but it is used for various tax and other business purposes. The Australian Business Number (ABN) enables businesses in Australia to deal with a range of government departments and agencies using a single identification number. The ABN is a public number which does not replace your tax file number. ABN registration details become part of the Australian Business Register (ABR) which the ATO maintains on behalf of the Commonwealth. You can access the publicly available information contained in the ABR through the ABN Lookup website.