Africa is great because it only has 3 borders but get 3 armies instead of one. It also has better expansion capability. Otherwise, you will end up stuck in Australia or south america wanting to expand to tough places.
Australia is good in 5 or 6 player game, only if you know that the opponents will play aggressively and that isolation would be to your advantage. The other problem is that if everyone in the game believes that the person starting from australia has an advantage, then you are already in trouble because they are going to fight you all the time.