As an Australian, I wouldn't object at all to New Zealand joining the Commonwealth of Australia (it was on offer back at Federation). I like New Zealand and New Zealanders.
However, I think that the two countries are significantly different. Australia is an old geologically-inactive continent with Aboriginal hunter-gatherer inhabitants from 40,000 years ago. New Zealand is a younger, much more geologically active country, with a Maori indigenous culture - and a much larger proportion of the population come from this indigenous group. New Zealand society is much more heavily influenced by the Maori people than modern Australia is influenced by Aboriginal people, at least in my experience.
The New Zealand government understands volcanoes and Maori culture. To Australians, they are exotic and "foreign". I tend to think that the special needs and situation of New Zealand would get lost in a "take-over".
I do think close economic ties - maybe something like the European Union - would be worthwhile. Perhaps a currency union. But New Zealand is often more advanced in its social attitudes than Australia is, and I (as an Australian) appreciate being able to point to New Zealand as an example of what we should be doing. So I can see economic advantages for New Zealand, but not necessarily any social advantages, in uniting with Australia. Something that achieves the economic benefits but leaves the social and political differences alone would be good.
I want Australia and New Zealand to be close friends - "family". But I don't want our distinct identities swallowed up.
Mind you, if I didn't live where I do now near Sydney, I would love to live on the shores of Lake Manapouri gazing at the view each morning ... at least during the summer. With global warming, I might even need to move to New Zealand to stay cool - maybe Dunedin!