An ABN (Australian Business Number) is an 11-digit identifier issued by the Australian Business Register. Every business operating in Australia must have one, and it must appear on every invoice you issue.
AusInvoice validates your ABN using the official ATO checksum algorithm — the same algorithm the ATO uses to verify ABNs. You can also validate any ABN using our free ABN lookup tool.
For more on invoicing without an ABN and the 47% withholding rule, see: How to Invoice Without an ABN in Australia.