Amazon don't/won't and never have shipped computer games or electronics to Australia, and I'm pretty no other major US retailer will, I assume it is to do with distribution lisencing from the publishers etc. (Why you'll see 'not for sale outside of country/region X on all games)
Back in 2007 there was similar issues with Steam not selling to Aussies new release AAA titles like Crysis because of the exchange rates at the time.
It would be nice to be able to save almost 50% at the moment on the price of the game also with the current exchange rates...