It's not about exchange rates and never ever will be. It's about purchasing power. It's just normal market segmentation, like how textbooks are cheaper in India/China than in the U.S.
Whats the average purchasing power of an Australain versus an American?
The Australian dollar is worth more than the US Dollar, and what's Australia's minimum wage? What's America's? They're about $15 and $7 respectively. If anything, Blizzard could probably stand to charge Australians even more.
But I'm sure they've done the math on this, and there might be other factors, but I'd gander that in terms of purchasing power the costs of the game to an American and Australian are probably pretty similar.
Now if you were making money in the U.S. and had to spend it in Australia, then you'd be very right to meow about exchange rates, since they don't take into account purchasing power, but very very few people who buy the Australian game are going to be in that position.