Australia and NZ could both easily have populations double or even triple what they are today given the right PoDs. A quick jiggling of numbers in a "no world wars" scenario that just extrapolates the non-occurrence of the demographic and economic losses would suggest a population of roughly 35 million for Australia, though that ignores butterflies. The main issue is a lack of immigration given structural and political barriers that are hard to overcome for internal reasons but quite possible with the right external developments. An easy one is just have something nasty happen to Britain that encourages enormous numbers of Brits to emigrate, easily could have half a million more migrants to the Antipodes or more.
A combination of factors could work too, say no/greatly delayed WW1 with the massive Antipodean casualties/missed migration opportunities are butterflied. No WW1 means an Antipodean population of probably 7-ish million by 1920, just extrapolating the early 1910's growth trends. It's important to remember how damaging the collapse in birth rates and immigration was during the war, not just casualties alone. Combine this with a later great war that ends with Britain seriously damaged and there's a great incentive to migrant down under, in particular that during the first half of the 20th century Australia was approaching the critical mass of population to undergo some proper industrialisation, which in turn creates its own growth incentives. One of the curious features of the 1900's Australian immigration features was a massive focus on agricultural workers from an exclusively Anglo background. Compare this to the 1950's policies which encouraged industrialisation, and which were much more successful. It's really not hard to see a 1950 population of around 11-13 million, which if it then grew at roughly historical rates would be around 30-31 million by 2017. Plenty of other butterflies could be put to use too, like a somewhat early to the White Australia policy driven by industrial interests for a larger workforce during the era that workforce size really mattered i.e. pre-automation. Even this is quite a conservative scenario.
RE "carrying capacity", Australia feeds around 60 million people on average per year, so agriculture is not a bottleneck to growth. Water is slightly harder, but only in so far as that without the political will to implement better policies the water supply is not increased. There are lots of very easy reforms that could be done to increase Australia's water supply, something likely to happen if people don't have enough to live!
A combination of factors could work too, say no/greatly delayed WW1 with the massive Antipodean casualties/missed migration opportunities are butterflied. No WW1 means an Antipodean population of probably 7-ish million by 1920, just extrapolating the early 1910's growth trends. It's important to remember how damaging the collapse in birth rates and immigration was during the war, not just casualties alone. Combine this with a later great war that ends with Britain seriously damaged and there's a great incentive to migrant down under, in particular that during the first half of the 20th century Australia was approaching the critical mass of population to undergo some proper industrialisation, which in turn creates its own growth incentives. One of the curious features of the 1900's Australian immigration features was a massive focus on agricultural workers from an exclusively Anglo background. Compare this to the 1950's policies which encouraged industrialisation, and which were much more successful. It's really not hard to see a 1950 population of around 11-13 million, which if it then grew at roughly historical rates would be around 30-31 million by 2017. Plenty of other butterflies could be put to use too, like a somewhat early to the White Australia policy driven by industrial interests for a larger workforce during the era that workforce size really mattered i.e. pre-automation. Even this is quite a conservative scenario.
RE "carrying capacity", Australia feeds around 60 million people on average per year, so agriculture is not a bottleneck to growth. Water is slightly harder, but only in so far as that without the political will to implement better policies the water supply is not increased. There are lots of very easy reforms that could be done to increase Australia's water supply, something likely to happen if people don't have enough to live!