Simply put it's the same reason a lot of video games cost more in Australia. Besides exchange rates, there are higher operation costs, marketing and a less competition-driven market makes things a bit ropy. Key to this though are regulation costs. Australia is practically legendary when it comes to censorship and giving games a higher rating than they would have overseas. Higher ratings drive up the costs as it often translates to fewer sales. It's why so many movie producers in America try to avoid the dreaded 'R' rating by removing or altering content.

It's not often something the publishers have any direct control over, so it's likely to persist as an issue for some time to come.