[quote=@Mihndar]
Well, they could take over Australia. That might provide them with significantly more resources and manpower to better be able to fight China if it comes down to it.

And also keep in mind Indonesia's population alone in 1980 is 150 million. Throw in Japan and South Korea, you've doubled that and you still haven't gotten to Papua New Guinea and whatever else they might own in the Pacific. Japan's military power would be nothing to sneeze at, considering they both industrialized before China and they have the numbers to at least compete. So no, they probably couldn't defeat China but it would hardly be a smashing defeat.
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I doubt the Koreans or the Indonesians would be at all loyal to the Japanese, or only really love themselves. The native populations wouldn't really want to support them voluntarily and the ashkaris they could pull from the islands wouldn't be worth much. Not to mention Australia is basically a whole lot of nothing and full of people even more opposite to the Japanese culture than the Koreans being raped or the Indonesians. So attempting to take Australia would just mean a lot of prolonged police work as well as attracting the ire of Britain, since Australia wants to and is trying to, or already has joined the Commonwealth. So they got some political protection there.

China as well would have it support from parties in the International. And since they'd act independently then they're not coming on Beijing's orders. Aggression on mainland Asia would also most likely attract universal ire from the Asian bloc who'd have enough power in the International Politburo to all declare war on Japan. They may not be able to contribute naval assets, but they could the land assets needed to turn a landing into a proper meat grinder while mainland Japan is burned from 30,000 meters.