Australia is extremely at risk of conventional invasion, their current independence is a function of alliance to the strongest navy in the Pacific. Without a US that is willing and able to ensure Australia's free access to the surrounding ocean, AU is absolutely unable to deploy enough of their own military to fend of probably even Indonesia, let alone China. The coastline is just far too long, the military assets too few, and the country too depopulated to be able to stop a determined invasion.
Indonesia lacks the force projection capability to even project an expeditionary force into Northern Australia.
Sustaining an expeditionary force into Northern Australia by Indonesia would leave it incredibly vulnerable to air and sea supply chain interdiction.
With first hand professional domain experience, and without arrogance or hubris, an Indonesian invasion of Northern Australia would be disastrous for Indonesia.
China invading Australia would entail a much more capable, but entirely untested, expeditionary force over much longer and far more vulnerable supply chains.
With just FVEY intelligence support and FVEY forces already forward deployed into Australia, the likelihood of China successfully establishing and sustaining a beachhead to break into Australia with a conventional invasion would be similar to that of Indonesia, due to very long and very vulnerable supply chains.
Unless China glassed Australia with nuclear weapons, any attempt by Xi and the CCP's PLAN/PLAAF/PLA to conventionally invade Australia would be a moon shot too far.
China's fleet steaming south would be severely attrited transitting limited maritime traffic route bottlenecks that would be akin to cattle chutes in a slaughter house, while China's own energy/food/raw industrial materials commercial maritime supply chains would be existentially vulnerable.
That's just to Australia's current fleet of Collins class submarines and tanker supported F35s.
Australia's AUKUS nuclear submarine investment will magnify that current independent threat to China's maritime supply chain.
Which is odd, considering this comedic skit is partially true:
https://www.youtube.com/watch?v=-cGYQneo-G8
Unconventional attack is far, far more likely. Thus requiring a focus on national resilience and adaptability to crisis.
Yes China has to transit the straits around SEA, but how many Collins does Australia actually currently have available to deny these channels, 1 or 2? Additionally, if this scenario happened and the US was in full turtle-mode, how long do you think AU could sustain those F35s? AUKUS won't deliver actual capability to Australia until maybe 2035 at the earliest, and those subs are too large to feasibly use the channels around Indonesia and Malaysia effectively anyway.
But yes I agree, unconventional attacks are more likely.
It’s happened before, and Australia has used discrete and unconventional means to disrupt it.
RAN could probably surge 3 Collins boats depending on timings of depot level maintenance.
P8 paired with C17/C130 used as arsenal planes to saturate PLAN air defence and F35 hitting hard targets with LRASM would make it a slaughter.
PLAN’s recent live fire exercise in the commercial air corridor between Australia and NZ single handedly justifies increased defence spending for ANZ.
Personally, I think China’s horrible demographic wall it’s about to hit at 100kph combined with a stagnant economy(140+ car makers today that will surely drop to 20 or less by 2035) leaves Xi with plenty of domestic crisis to solve.
The risk is if Xi needs(or needs to create) an external crisis to activate nationalism and deflect away from domestic strike(akin to Argentina-Falklands 1982).
Even Taiwan might be a stretch too far. Xi will need a guaranteed win.