Australian wages are on average (and in US dollars) DOUBLE what the average US wage is. While the conversion rate of the currencies are about the same, the sheer amount of money each person earns is vastly different. Basically your $110 games is the same as an American getting the same game for $55 when you figure in the differing monetary grosses of each person.
Quickly, US gamer earns $1 for something while the Aussie earns $2 for the same thing. The differing prices are set to make them equal to each countries SOL (Standard of Living) so that a person doesn't import all of their goods, which would cripple a country in the long run if enough people did it. It's the new Mutually Assured Destruction that is based around economics instead of Nukes.