What would happen if China attacked Australia?
With non-nuclear weapons?
Well, about 6 months before the planned invasion, news would get out that the Chinese are planning a massive invasion somewhere. You can't achieve that level of readiness without tens or hundreds of thousands of people getting involved, it is impossible to keep a secret.
The assembly of the invasion fleet would be continuously monitored by satellite. The US Pacific Fleet would move into the South China sea.
The progress of the Chinese fleet would be monitored, and of course by this stage Australia would have moved its naval and airforce into position.
Nothing much would happen until either the Chinese directly attacked Australia with cruise missiles, or came within 200 km of the coast.
When they do, the Americans would probably say "have some fun, guys" to the Australians. China has no aircraft carriers. Australia would enjoy total and undisputed air superiority. It would be a complete turkey-shoot. The first target would be the troop ships. When they are sunk, the Chinese will probably lose interest. When the rest of their ships are sunk as well, they definitely will.
China has nowhere near the ability to project sufficient force to contemplate taking on Australian forces near Australia. Except for the US, nobody does. You would need several very large aircraft carriers to even come close to a fair fight with the RAAF, and only the USA has this capability. And until you take out Australia's airforce and submarine fleet, nobody is going to be sending troop ships to Australia.