An ABN (Australian Business Number) is a unique identifying number issued to businesses in Australia. You can obtain an ABN if you are "carrying on an enterprise" in Australia.

If you send someone an invoice but do not supply an ABN then they are usually required to withhold tax from the payment to you. The amount they withhold is normally about half of the invoice amount.

The tax office provides a good overview of what constitutes carrying on an enterprisefor the purpose of getting an ABN. Normally if you are carrying on an enterprise you should get an ABN.

Your ABN is published together with your legal name and postcode on the Australian Business Register. You need an ABN:

• in order to register a business name;
• to register for GST (which you are normally required to do if you have a turnover exceeding $75,000 a year); and
• in order to have invoices paid without tax being withheld by the payer.

Your ABN should be placed on most business documents, for example contracts, invoices and receipts.