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 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.
You can check if a business has a valid ABN here