Whose responsibility is it to ensure the accuracy of ABNs?

In general, if the ABN quotation looks reasonable, you can accept it. However, if you have reason to suspect that it might not be genuine, it does not belong to the supplier who quoted it, or the supplier has incorrectly charged GST, then you should check the Australian Business Register (ABR) to verify that the ABN is valid and if the business is registered for GST

www.abr.business.gov.au

The ABR lists entities that have ABNs whether or not they have GST registration. Those that have GST registration show an effective date. If an enterprise is not registered for GST, then it cannot charge GST on its supplies and consequently will not (or should not) issue a tax invoice including GST.

You can only claim GST credits on purchases from suppliers who are registered for GST.

If the ABN quoted on the invoice is not valid or the details do not match the supplier, you should withhold 46.5% of the payment.
There are some specific circumstances where you do not need to withhold and you are able to obtain an 'ATO Statement by Supplier' declaration as an alternative.
This is an area that can cause headaches if it is not handled correctly, so if you have questions please contact Peter Flemming on 07 3878 8889 or peter.flemming@schuhgroup.com.au