I know this thread is now dead, and Elvis has left the building, but I just want to clarify one more statement above. The Aunt Nellie example is not correct. The very first exemption to deducting witholding tax when no ABN is quoted is where the "payer" (as opposed to the payee), "is not making the payment in the course of carrying on an enterprise in Australia". Aunt Nellie does not have to deduct witholding tax.
Once again, DO consult a Tax Accountant.