I wouldn't be surprised that Australia could be on a wish list for invasion , if it was possible . Raw material rich and population poor makes us a target . The only nation continent .
This is why we've cultivated the US as an ally . Conventional as well as nuclear protection . In their interest that we remain free as well as our own interest of course .
In a conventional war it may be possible some time in the future ...certainly not for the forseeable future though because of US strength . Easier to defend than attack too . All the invading forces have to come a long way by sea and air and are good targets while doing so .
Forget nuclear as a method of invasion , the world is so saturated with nuclear weapons that adding more has no effect ...MAD prevails and no-one wins in a nuclear war . Only extreme desperation/madness allows use of high level nukes .