In a way, the Fortress Australia strategy is one of the many calculated stupidities found in every Risk player's playbook. If you decide to go with the FA strategy as your play for the entire game, you would begin by building up in Indonesia or Siam early on and slowly expanding out into Asia while the rest of the world fights among themselves. More often, FA is used as a desperate punt: pull back to Australia and attempt to stop the onslaught. Then your oppressor might move on to eliminating someone else while you recover and begin to expand again.
However, using FA in either of these ways usually ends in the same result: your death. Sometimes you get lucky, but let's face it. You get 2 armies for Australia and no more continent bonuses unless you conquer Asia, which is a large feat for a player owning Australia and a couple of Asian countries. Meanwhile, other players get their huge North America and Europe continent bonuses, conquer four more territories and South America is in their hands for another 2 armies. The other armies will make their small limited strikes and wipe out anything you gained in your impossibly risky grab for Asia.
So what's the solution? Use FA in conjunction with another strategy. To continue with the football analogy, don't use FA as your only play or even as a 4th down punt. Claim South America or Africa as well at the beginning of the game and make your small raids out of these continents, slowly expanding for Europe and North America. Meanwhile, while you make your three-yard running plays from Africa or SA, make the incredibly deep pass for the touchdown. Spare a few of your ever-increasing armies each turn and put them in Australia. When the time is ripe, lash out and begin to take Asia. The worst thing that can happen is that you don't get Asia, and you continue with your other, more conservative strategy on the other side of the board. But if you do succeed in taking most of Asia in a quick raid, just hope for a Risk Pro Bowl.