Australian Business Number

The Australian Business Number (ABN) is a unique 11-digit identifier issued by the Australian Business Register (ABR) which is operated by the Australian Taxation Office (ATO). The ABN was introduced on 1 July 2000 by John Howard's Liberal government as part of a major tax reform, which included the introduction of a GST.
The law requires each entity that carries on a business in Australia has an ABN and that the ABN appear on each tax invoice and other tax related documents issued by the entity.
Australian Business Register.
The Australian Business Register (ABR) is maintained by the Registrar of the ABR, who is also the Commissioner of Taxation. The Registrar registers entities, issuing them with an ABN, while the Commissioner of Taxation issues the entity a tax file number.
Entitlement to an ABN.
The Registrar issues ABNs only to entities that are entitled to an ABN, which can be:
For an entity to be entitled to an ABN, it must:
Whether or not an entity is carrying on an enterprise is a question of fact and there are many circumstances where an entity will be carrying on an enterprise. Without being exhaustive, an entity will be carrying on an enterprise if it:
The Registrar can refuse an entity's application to be registered. Equally, the Registrar can cancel an entity's registration and thus their ABN. Each of these decisions are reviewable taxation decisions.
Applying for an ABN.
An entity can apply for an ABN:
Before applying for an ABN the entity must have a tax file number (TFN).
Format of the ABN.
The ABN is an 11-digit number where the first two digits are a checksum. Unlike with the tax file number (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 the two-digit checksum is prefixed.