The Chinese could parachute in maybe 10,000 paratroops in one go. That's being very generous in allowances, more realistic would be 4-5,000. Then they have to fly back and get more. That's flying all the way from China or Burma. Even that fleet of 60 ancient and slow soviet era transport planes would be easy to spot and probably at least 8 hours advance warning and that's if they could organise that many troops and aircraft without anyone becoming suspicious.

They could build or buy more transport aircraft but that would take a long time, be very expensive and also be very obvious. You just can shift that amount of troops that far in anyway short of some teleportation method without a shitload of advance warning one way or another. Not to mention they'd have to go over quite a few countries first, most of which are technically our allies.

China's military is mostly structured around defence and making any regional conflict very painful for the US Navy and allied countries like Japan, Korea and Taiwan. Also China has potential threats in India and Russia on it's borders, Pakistan is a mess, Afghanistan even worse, even the other SE asian countries dont particularly like China that much, the Chinese even tried to invade Vietnam in 1979 and got a dose of pain that made them think twice. The only two allies China really has are Burma and North Korea, both of whom are basically useless and serve to piss off other countries. China would have a hard time invading anyone without pretty much pissing off all or most of it's neighbours and in fact most of the world.

Now of course if China really wanted to invade Australia, they probably could, but they aren't idiots, it's just not worth it, not while we happy sell them all the resources they want a reasonable enough price and then they can make shit out of it and sell it right back to us and most people are making money so everyones happy. They get to use their money to boss around some crappy african countries so they still feel important, they know they can have the Americans at least thinking twice about intervening in something like Taiwan, but they also know going to war over it would basically make it pointless to own.

Also I think you underestimate Australians ability to be guerrilla fighters, especially if you tell them there are foreign immigrants trying to jump the queue ;)