The biggest threat to Australia isn't invasion, it is our economy.
Our biggest trading partners, the ones we actually have trade surpluses with, are China, Japan, South Korea and Taiwan. Any war involving China, whether we are on the winning side or not, will be economically disasterous for Australia. The US will be fine, they can provide most of what they need by themselves and are bordered by "friendly" nations such as Canada and Mexico. Europe is also its own ecosystem. Even China could probably rely on Russia to gain access to the resources it would need to keep its wartime economy afloat. As for Australia, we are probably screwed. Protecting our SLOCs may be pointless if our trading partners don't exist. The Australian economy would be lucky to survive more than a year or two.
To make things worse even if Australia stood on the sidelines our economy would still be screwed.
The best thing we can do is create alliances in the hope that this will be enough to deter Chinese adventurism.
This should be enough to keep strategic planners laying awake at night.