I’d expect them to stay out of it at this point, actually. They want to expand into the pacific, and the US and East Asian democracies want to stop them. Meanwhile, Australia also has affinities with now-distinct Europe, isn’t directly in the way of any of that, and depends heavily on China for trade.

And, even if it did become involved, dealing with a Chinese occupation isn’t going to be as hard as a nuclear winter or the total breakdown of modern civilisation.