You could easily have quite a late PoD, right up to the mid 20th century. If Britain and France are both on the losing side of a major war it's entirely possible that Australia could take responsibility for both their Pacific empires, especially if for whatever reason the US is not present in that part of the world. From there it would not be difficult to make NewCal into some sort of autonomous dependency of Australia, perhaps if labour mobility from the Pacific to the continent became possible. It would almost certainly be easier if NZ had joined some form of Federation.
The South Pacific being independent from Australia was as more due to the lack of Australian desire to incorporate than any serious nationalism on the part of the people living there*; if Australia has a reason to want it, it will probably get it.
*Obviously this is very different today, but even a brief glance at the political history of Fiji, by far the most politically sophisticated Pacific state, shows that the Pacific Islanders themselves did not want independence from Britain even up to the 1970's. It is also quite probable that the Papuans would have, if given the chance, voted to stay with Australia in 1975. The New Guineans not so much.