Every time I've played Risk, two things happened. First, a massive bloodbath over Australia. Second, a long slow slog to a dissatisfying end, where multiple players wind up pissed off, because the Australian ends up winning somehow.
The AU continent is just so easy to hold. With only one country to go through, you can amass a huge army in Siam and no one touches your continent all game.
AU is an easy way to get second place, not to necessarily win. Focus on getting cards elsewhere. Card bonuses build up to where the Siam bottleneck won’t matter.
While everyone else is fighting over Australia, I generally managed to get a win by completely ignoring it (and uh, making use of the people that were so irrationally focused on it).
The AU continent is just so easy to hold. With only one country to go through, you can amass a huge army in Siam and no one touches your continent all game.