Well here's my point:
The way the "post-atomic horror" of Star Trek was presented to us, it seems humanity was taken to the brink of annihilation, only to then be wrestled back from all of that by the twin fortuitous circumstances of Cochrane's warp flight, and it's unintended consequence of bringing the Vulcans to our world.
All of that happens in Arizona, USA.
If the planet as a whole has become something of a wasteland (and it'd have to have been a pretty big bang to make
that happen), and has maybe seperated into isolated enclaves even across a vast land-mass like the United States, then Australia has got a
natural geographical isolation that makes it one big enclave from the whole rest of the world.
(I speak from experience *as* as Australian: this is almost the truth even in today's world.

)
So, assuming all the other countries have blown themselves to smithereens, or even that Australia has taken itself down with them, chances are pretty good that in any case when the northern hemisphere countries all come a-knockin' on our door, saying "Gee, we're sorry about all that post-atomic horror stuff, but we've got this swell idea now for a United Earth, we've managed to sign up most of the planet but we want to know if Australia is on board?", there's
at least a 50/50 chance whatever is left of Australia's parliament is going to take their sweet time before saying 'Yes'.
As an Australian, I've never once questioned Dr. Crusher's apparent statement that Australia was one of the last governments to sign up to "United Earth". I can buy that 100%.
