Check an ABN before you pay

You can, but you may have to withhold tax. If a supplier doesn't quote an ABN and the total payment is more than $75 excluding GST, you generally withhold the top rate of tax from the payment and pay it to the ATO (ATO: Withholding if ABN is not provided (opens in a new tab)). The top rate is 47% (ATO: Foreign resident PAYG withholding (top rate of tax: 47%) (opens in a new tab)). The same applies, with the same exceptions, when the ABN on the invoice is not valid or the details don't match the supplier (ATO: Checking an ABN (opens in a new tab)).

If you withhold, you also give the supplier a payment summary, and you can't claim a GST credit for that payment. If a supplier has applied for an ABN, you can hold the payment until they quote it, but you must not pay in full on the promise of one (ATO: Withholding if ABN is not provided (opens in a new tab)).

When you don't withhold

The ATO lists payments you don't withhold from, whether or not an ABN is quoted (ATO: Payments you don't withhold from (opens in a new tab)):

• the total payment is $75 or less, excluding GST (you can't split a larger job to get under it);
• the supply isn't a business transaction, for example a private, domestic or hobby supply by an individual;
• the whole payment is exempt income for the supplier, as it is for some charities and community groups;
• the supplier isn't carrying on an enterprise;
• the supplier is under 18 and you pay them $350 or less a week;
• the supply comes through an agent who quoted their own ABN;
• the supply is wholly input taxed.

When one of these applies, the supplier can give you a written "Statement by a supplier not quoting an ABN", and you keep it for 5 years (ATO: Payments you don't withhold from (opens in a new tab)). About the "not a business transaction" exception, the ATO says: "If your business is a partnership, company or trust, and the supply is made through your business, this exception will not apply." (ATO: Payments you don't withhold from (opens in a new tab))

Every ABN has two check digits built in. The ABR's method (ABN Lookup: Format of the ABN (opens in a new tab)):

1. Subtract 1 from the first digit.
2. Multiply the 11 digits by 10, 1, 3, 5, 7, 9, 11, 13, 15, 17 and 19.
3. Add the 11 results.
4. Divide by 89. If nothing is left over, the number is a valid ABN.

The ABR's own example, 51 824 753 556:

The box at the top does this as you type. A passing checksum only means the number is well formed; the register lookup is what shows whether the number is on the register and whether its ABN is active.