So take it from a young man who spent way too much of one summer playing Risk on the computer, you don't want to take Australia, the problem with Australia is that there is no good way to go from Aussie to anywhere else because the only thing next to you is Asia so someone else can solidify gains somewhere else, and Aussie only gives you 2 more reinforcements, not enough to get you a decisive enough edge to move out of Asia.

The trick is to always capture SA, it is close enough to other things to keep you involved, also you can capture Centeral America and North Africa without having to hold any more territories than you would need to. Then choose NA or Africa and work on seizing the rest of that, when you complete that if it is late enough in the game you'll win almost every time because Asia, and Europe are impossible to hold, North America is to big to capture in the early game and Austraila as noted before doesn't bring a big enough advantage because it puts you in a poor tactical situation.

EDIT: A good point was made below, this applies only to the standard classic Risk map, in standard Classic Risk.