Hi Shipmate,
With that budget and if you want to get close to $ 300, go for the ATI Radeon HD HD 6970. There is no card beyond this from the Radeon stable !!
I am not aware of any Australian website but you can check out the American one called www.newegg.com just to get an idea of the cards available in your budget range. Of courses, prices may differ.
Regards
vin_sun