Although these ideas will have to slot into how your war starts, I think it would be easy enough for US personnel to be stranded in Australia.
Some ideas for how they ended up in the Great Southern Land
1. They could have been separately in the country on holidays and met each other at the airport when trying to get back to the USA. Upon finding out nobody is flying anymore they stick together due to shared nationality. Alternately they could have been passengers on the same cruise ship and then find the ship cannot leave.
2. They could have been a group of friends on an adventure holiday and like No 1, find they cannot leave due to the war.
3. The PCs might have been embassy/consulate staff (or family of staff), stuck in the country when the world war kicks into high gear.
4. They could be a business group in Australia for work purposes and get caught in country when the world goes pear shaped. Alternately they could be from a defence contractor corporation sent to Australia to assist in Australia purchasing military equipment from the US (e.g. General Dynamics has subsidiaries in Australia).
5. If they are military they could have been in the country conducting peacetime training with Australian forces or been advisors for a training package for Australian forces.
6. They could from a group survivors of some war related disaster evacuated to Australia because it was the closest country that could deal with the refugees medical needs (or it was the nearest friendly country).
7. They could be remnants of a military task force that was sent to Australia to be reconstituted but as the world turned to crap, the reinforcements are never going to arrive and the commander turns them loose to make their own way back to the USA - Good luck, you're own your own...
As for Australia itself, it is a "bloody huge" landscape with many variations in terrain and vegetation. Everything from dried-out lakebeds to tropical forest/light jungle, mangrove swamps, dry forest, coastal cliffs, vast deserts and also mountain ranges that get cold and wet enough to have snow.
Here's the first website you should read
http://www.dfat.gov.au/aib/overview.html
It's rather long (6 pages I think) but provides a better overview than most websites.
Threads from this forum that might prove useful for information
http://forum.juhlin.com/showthread.p...ight=Australia
http://forum.juhlin.com/showthread.p...ight=Australia
http://forum.juhlin.com/showthread.p...ight=Australia
http://forum.juhlin.com/showthread.p...ight=Australia
http://forum.juhlin.com/showthread.p...ight=Australia this is the "biggie", 8 pages long but well worth the read