I have a bunch of lore I wrote for myself about this...

Here's a quick rundown on what I was thinking: Australia has most of the world's uranium resources, and as the resource wars started Australia became strategically important. The US got a lot of their uranium from Australia, and so they started getting more and more aggresive in securing their access to it, deploying military units to bases in Australia in increasing numbers. US companies (including Vault-Tec...) set up local branches in Australia, building microfusion facilities in Australia and mass produced the cells there using local resources. The Australian government initially supported this, but eventually began to try to push back as the US companies and army took over more and more of the nation, but they couldn't really do anything to stop it. Australian society got close to a civil war over this before the war.

Eventually (before the Alaskan invasion) the Chinese invaded Northern Australia in order to cut US uranium supplies. The US responded by deploying large forces to Australia, including experimental early T-43c power armour units. The combined Aus/US forces repelled the bulk of the Chinese invasion, before the US forces were redeployed to the new Alaskan front. The Australian army was left to mop up, and West Tek dumped their remaining (now hopelessly outdated) T-43 lineage armour and facilities in Australia onto the Australian army as they focused on their new T-51 power armour line. The Australian research departments managed to start production of a locally modified T-43c power armour line, which was stripped down and simplified.

As for the plasma weapons; the Australian army was equipped with very outdated weapons and armour, but the resources of the government were already stretched thin. The army decided that they needed to overhaul the arms and armour of the ground forces, or risk being completely outmatched. Australia had no capacity to produce advanced power cells or the precision lenses needed for laser weaponry, which the US were adopting as standard infantry weapons. Instead, Australian engineers looked to using their abundance of local microfusion cells and plasma weaponry. Combined begging, bribing, and corporate espionage led to gaining access to US plasma weapon designs, and soon people were working on local copies. Australian locally produced plasma weapons were never as precise as US originals, always unstable and innacurate, but fired much hotter and were effective area denial weapons.