Really like the look of the Boomerang III!
And the Bolingbroke is a fair contender for the ground attack role, although I would be more likely to go with a modified Beaufort, which Australia was already producing & which was faster than than the Bolingbroke (Australian produced aircraft were, apparently, even faster than the British version).
It still comes down to what a country of 7,000,000 people can do against a country of 73,000,000.
As an Australian, I'd like to say "Kick their @rse$!" but, unless the Japanese were hampered by active, aggressive resistance in the occupied territories of SE Asia I have a horrible suspicion it would not go quite like that.
Firstly, the Japanese were well aware of the benefits of a strong navy to a nation relying of the sea for bringing in supplies for its people & industries. They were aware of what they had done to the Russians in the early years of the 20th century & of what the British had done to the Germans during WW1, &, somewhere along the line, they obviously picked up an understanding of the need for naval airpower. All of that would have to go. (Remember: the Japanese were not afraid of US battleships, the whole idea of Pearl Harbour was to take out the USN's aircraft carriers. Question: If they had done that, would the US have done what the Japanese hoped for - pull back to the Eastern Pacific & maintain a defensive stance, at least for long enough for the Japanese to gain a firm hold on SE Asia & the Western Pacific?)
Secondly, you'd have to take Yamamoto out of the picture completely, he was a skilled proponent of a strong navy, with influence in political circles, in the pre-war era (& not just because of America) & one of the architects of Japanese naval power.
Thirdly, even in the SE Asia /Indochina route you have given, there is still a lot of island-hopping to do to get to Aus, so, once again, naval power is important.
Sorry, I guess I just can't see the Japanese not having a serious navy.
I suppose you could have them being as lax as Australia was (more-so "
is") in the understanding of the need to defend its ocean life-lines without a major ally to assist. Actually, during WW2, Australia had the navy it could man, by the end of the war the RAN was larger than Australia could cope with & many of our ships had cadres of British sailors & officers to flesh out the numbers required to crew them.
Where would you expect the Japanese to land? With the GBR protecting much of the Queensland coast from unrestricted access I would expect them to land either in the SW of the Gulf of Carpentaria, near Karumba/Normanton, or (more likely) south of Rockhampton (say Bundaberg). Darwin is also an option, with the idea of moving south to Adelaide & cutting the country in two, with a possible follow up landing to take Perth (thus effectively "capturing" the entire western half of the country) before heading east.
Guy