GDP about 35th in the world compared to NZ on about 53rd, the worlds 5th largest air force, large high-tech manufacturing sector with significant exports, space programme, nuclear weapons with independent launch capability, independent alliance network, significant efforts toward self sufficiency in resources. All of that sort of thing.
Not trying to derail the thread, but... Israel has the 5th largest Air Force? Sh*t, that's impressive. The comparison is rather illustrative.
Now, back to the OP, does having New Zealand in some kind of "union" or federation with mainland Australia counts (and Australia being the "junior partner" for some reason)? Perhaps both Australia and NZ are integrated into the same colonial administration for some reason, and then Australia despite its enormous size suffers some issues that allow the transplantation of the center of decisionmaking to NZ... that's far-fetched, I know (much like having the UK administered from Ireland or the Austrian Empire from Romania), but let's try to keep an open mind, considering its an AHC.
Let's say, for example, that settling in Australia is less interesting than in NZ (indeed, we can compare the ratio of available land in the whole of Australia, with but New South Wales and the western tip having bigger population centers), and somehow even these more settled regions in Australia suffer widespread depopulation and migration towards NZ (just to work with an example, an even more aggressive Japanese attack, but now we might be slipping to a "Man in the High Castle" setting, I admit). In due time, with the correct administrative and economic developments, NZ might become a Switzerland or Nordic country of sorts in the Pacific zone, and with a more sizeable investment in modern weaponry, which dispenses huge manpower (such as a Navy), it might become a formidable power.