Speaking as an almost ignorant visitor:
Immigration is not an issue, the point system will take care of that. Salaries in NZ are much lower than in Australia, and it seems people jump to Australia if they can. Auckland is great (if a bit industrial in areas) but once you've seen the country, going anywhere means taking a flight and $.
Australia taxes are high (and permanent, taxed on world wide income if you become a citizen) and salaries for professionals seem to have a narrow range so that there is a strong middle class, but you need to be in finance to reach salaries in the 6 figures.
Sydney is amazing, but very expensive. Melbourne is a nice second and more affordable, but salaries are also lower. We didn't checkout Brisbane.
Great vacation, not so sure about leaving the US for it.