Exactly, looking at how much it costs do just download the game should remove any local influence on the price or cost to get a box there. It should be equal to the US price because of the exchange rate. My guess is the number of digital copies is limited, restricting supply.
But you found it for $51AUS?! Link your fellow Australians.
I have used this website before, as have many of my friends and had no troubles (even with Origin keys), but obviously do so at your own risk etc. etc.