Churchill said-
"In war, one has neither friends nor enemies, only interests"
so
IF a future American administration thinks it's in her interest to help Oz you've got zilch to worry about, otherwise you'll be up the spout if you've already cut loose from Britain..
I'm not sure why you're suggesting Britain is any more immune from being a fair weather friend than the Americans would be, but a cursory glance at our strategic situation would suggest the Americans are far more invested.
That's not a knock on Britain in any way. But, since in war one has neither friends nor enemies, only interests, it behooves us to act with forethought and planning, rather than relying on colonial bonds which haven't proved at all useful in protecting our interests and borders since...erm...
When did they EVER prove effective in protecting our borders and interests?
Meh, anyway. The thought of Australia relying on Britain for anything these days is ludicrous.
China, Japan, the USA, Korea...heck, even Singapore are larger economic partners to Australia.
And whilst there are some useful ties between the UK and Australia in a military sense, our US alliance dwarfs all others in terms of importance. We have agreements with various Asian countries, since that is the region which effects our border security.
On a global scale, the UK, US and Australia have generally acted in concert of recent years. That's not because of ties between the UK and Australia, but there are obvious commonalities in global view at the moment (at least).