When you pay for goods or services, the supplier generally needs to quote an ABN. They can do this on an invoice or some other document that relates to the goods and services.
If a supplier does not provide its ABN, you may need to withhold an amount from your payment. This is referred to as 'no ABN withholding'. The withholding rate is currently 49%.
Certain suppliers are not required to quote an ABN. These suppliers can use a Statement by a supplier form to justify the payer not withholding from their payment. To get this form and further information, search for 'Statement by a supplier' on the ATO website.