The below stated is correct but Australia should not be stated in a way that seems that Australia belongs to the British empire as it is an independent nation. The Gaijin should add a New Zealand planes also, as a part of premium tech tree.
In War Thunder, Australia is portrayed as part of the British Commonwealth and, as such, aircraft and tanks from Australia are included in the British tech-tree. Australia has some aircraft, just 10 of them, with only 2 being researchable and the other 3 being premium aircraft. It also includes 2 tanks, but in premium tech tree.
On the War Thunder live site, there are aviable Australian skins, mostly made by Aotea. Also, some skins are unlockable, like the one for B-25 Mitchell light bomber. But if you want to add custom skins on your vehicle, you will need access to files and program your skin to show on game. It takes some time, but it is worth it.
Australian planes include :
The Boomerang Mk.I
The Boomerang Mk. II
The Beaufort Mk VIII
The Thunderbolt mk. 1 ( currently replaced by Thunderbolt with British markings, but still available to anyone who buyed it before)
Australian tanks include :
The Matilda Hedgehog
The Sentinel aka Thunderbolt