Hmmm i could picture a few things.
1. It would obviosly extend the war in the pacific if the US decided to liberate Australia and New Zealand.
I think it would be sometime before Great Britian could do anything about the fall of Australia and New Zealand...same as how it took until late 44 to mid 45 for the British to be quite active in the Pacific again.
Since it would require some time to liberate these areas and quite a bit of men and material i think of this option as the least likely because i think i would have taken from forces going to Europe.

I think the more likely situation would be the US competely bypassing that area of operation if the area was captured. Fighting it's way to the Philippines through the central pacific only. Attempts to rid the area of Japanese would not occur until the very last months of the war primarily by British forces.

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Never, never, never believe any war will be smooth and easy, or that anyone who embarks on the strange voyage can measure the tides and hurricanes he will encounter. The statesman who yields to war fever must realize that once the signal is given, he is no longer the master of policy but the slave of unforeseeable and uncontrollable events. -- Sir Winston Churchill