Anyone who has a read of my comments has probably noticed a common (tedious) theme along the lines of "I wonder what that would look like in Australian service in that theatre, during that period etc etc".
Well I think I have come up with a way to do it, a common adaptable back story that justifies almost anything I want to do.
Japan catches and damages some of the USN carriers during the attack on Pearl Harbor and goes on to win the Battle of the Coral Sea and the Battle of Midway. This dramatically reduces US combat power in the Pacific while leaving Japans mostly intact. While not changing the outcome of the war it does take the US longer to wrest control of the Pacific off the IJN and more importantly gives Japan time to take New Guinea (with the successful invasion of Port Moresby negating the need for the overland attack) and subsequently invade Australia (North Queensland) in late 1942.
Long story short, the Brisbane line worked, Australian forces were able to hold in particular with the support of the armoured divisions intended for service in North Africa. At the same time the Light Horse, renamed and re-equipped by this time as Motor Regiments and Brigades, were able to harass the enemies rear areas. Miraculously Darwin and Broome held as well with line of communication, not only maintained but improved. Japans invasion of Australia drained resources from the Pacific Island and Burma campaigns freeing up US and Commonwealth forces to aid Australia, New Zealand and South Africa in particular went above and beyond in their mobilisation to support Australia.
The biggest issue was airpower, or more to the point lack of combat aircraft. This was rectified from early 1943 but it had been a close run thing with many civilian casualties resulting from air attack and the RAAFs almost complete inability to defend local skies. While the delivery of Spitfires enabled the RAAF to defend population and more importantly industrial areas it was the arrival of veterans of the Desert Air Force, flying P-40s and then F-4U Corsairs that turned the tide of the air / land battle.
The fight back was long, slow and bloody with the last Japanese forces on Australian soil being overrun in late 1944. As a matter of pride Australia led the invasion and retaking of New Guinea before insisting on involvement in the island hoping campaign and the inevitable attack on the Japanese Home Islands. Both Implacable Class carriers were commissioned into the RAN and Australia withdrew RAAF personnel from Europe to join the attacks on Japan with the crews switching from Lancasters to B-29s and RAF to USAAF operational command.
The fall out from the invasion of Australia was extensive, it saw the end of Robert Menzies political career as he was blamed, rightly or wrongly, for leaving Australia undefended two years into a world war. In true Australian tradition there were a series of Royal Commissions into how the invasion had been allowed to occur and how Australia had been so unprepared. Having suffered an actual invasion as opposed to the fear of one meant that real questions were asked and real answers were sought. Jellicoes recommendations were examined as were those of Monash and Chauvel all from the post WWI period, most tellingly (and recently) those made by Richard Williams were assessed as well. The findings, unsurprisingly, were that had the government(s) of the day listened to these respected leaders and made an attempt to adopt their recommendations, or at least to compromise on them and for an expansion base then Australia would have been prepared for the invasion, forecast decades earlier and would likely have repelled it.
How to respond to the findings and how to address the issue of national defence when key allies are other wise occupied caused much discussion and argument. The States that had been left outside of the Brisbane line were furious and insisted on their own means of defence, the public in general were horrified that the UK had been so slow to release the 2nd AIF to return to Australia and had been even more reluctant to release embedded RAN and RAAF units and personnel.
The result was a series of major changes to the Australian military. First and foremost a regular army was formed serving along side the RAAF and RAN which together formed the Australian Defence Force (ADF). The ranks of the ADF were filled by a mix of regulars, conscripts and reservists and the force was legislated for use within Australasian and South East Asia only for Australian defence only.
The next level was a State based National Guard, formed from the old Militia and reporting to each of the State Governors and through them to the Governor General, not the Federal Government. Each State had a balanced brigade to division sized part time force (depending on the population of the state), including Cavalry Infantry and Artillery as well as an Air national Guard Wing for air defence and civil defence.
The final Element was the Australian Expeditionary Force (AEF) the spiritual successor to the 1st and 2nd Australian Imperial Force(s) (AIF). This was made up of a number of Regiments of variable size (depending on strategic need), RAAF combat Groups and RAN Blue water squadrons, whose elements could be used to form combined arms task forces for international deployment in support of the UN or Allies.
Very messy and very stove piped but it lets me basically do almost anything as Australian Military