Hi stmstr,
If you have an Oz licence you should be ok theory wise, you don't say if you are mechanical or avionic. The exam is heavy on electrics, (well it was back in 18 fish and chips). Just be careful with some US 'translations' of the English.
Regarding taking the exam outside the US, check with your local FAA office. There must be one in Sydney, (sorry, I haven't got my directory to hand). They will be able to give all necessary info.
Good Luck
JA