If you want something sent back home to yourself from Australia and, like me, you find the jet-propelled carrier pigeons unreliable, do as Rory says, only put Aussie stamps on your envelope. So easy!
Here is the link to
Australia Post International where you can find out how many stamps worth to attach. I looked. It's $2.20 (Aus. dollars) to send an ordinary letter to the US.
I'm happy to send anyone a book of Australian stamps if you can't buy them online!
Cally