Since you have the "North American" maps on your Go 720, you'll have to download Tom Tom's HOME software onto your computer so you can buy the Austrailian map. Once you buy it you can transfer that map over to your GO Gps unit. I suggest you get an SD card to expand the memory so you can save the map(s) on it and use the built in memory for actual navigation. It responds faster if you do that.
It sounds like you accidently selected your American map on your GPS then. If you have the Australian map on the GPS you need to select that one.
Your menu should have a "manage maps" button on it, and if it does not appear, you need to select "Show all menu options" to get it to appear.