Since most of the products sold at Amazon are delivered only in USA, you have to get your own USA Address. Get your own USA Address from Shipito.com. This is where Amazon will deliver your order and Shipito will then ship it to your address in Australia.
Wait for a confirmation from Shipito. They will send your address via Email. The address is free but you have to pay $8.50 for every package that you send out from your USA address.
You can immediately start shopping once you receive the address.