In attacking NZ and Australia do they use troops they had in China, Burma, or the Philipines? In any case cut off the head (Japan) and the body will die. Taking the Philipines back wasn't necessary and following the same route to Japan would have ended the war as it did after the dropping of the bombs. The troops in Australia and NZ would be cut off from being relieved or transported back to Japan for its defence. Japan was having a hard time reinforcing the islands and was forced to leave them there. I do believe the Australians as well as NZ would have put up some stiff resistance and it would depend on the determination of the Japanese and their evaluation as to the strategic importance.
IMHO
