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.