ABN stands for Australian Business Number and means that you are registered as part of the Australian Business Register (ABR) which is maintained by the Australian Taxation Office. The underlying purpose of an ABN is to allow for one single identification number for a company when it deals with government departments and agencies.

ACN stands for Australian Company Number which are issued by the Australian Securities& Investments Commission (ASIC). When a company registers under corporations law, it is issued an ACN.

Whereas an ABN has 9 digits, an ACN has 11.