I'm fascinated by the idea, because I believe consolidation into larger units carries a lot of merit in certain parts of the world. (There are, however, limits.) What is the benefit of being a tiny country?
I think it would be a good thing, economically, for New Zealand, and it could be a model for others to move towards greater cooperation. Also, it will piss in the eye of realist theory, which I can't help but endorse.
(Side note: The last two occasions I can think of another country just up and becoming part of another country would be Hawaii and Texas with the US, and I'm not sure how well they count, since it was after an American coup in Hawaii and an American-led revolution in Texas.)