I'd choose Australia over NZ. Beta is correct about 'small, parochial and stuck in the 1950s', if you go outside towns or cities of any size. It's lovely to look at, granted, but the economy's 'not in a good place'.
I've never lived in NZ as such, just visited, though that time combined equals a year. I have a lot of friends there but the thing that strikes me as I visit over the years is how their- well, though processes slow down along with their speech. One close friend, previously intellectual and quite ambitious, told me that where she lived, she'd far rather her 12 yr old DS made the first cricket team than got As at school as the former would confer school glory (and stop him being beaten up by the Maori boys!), the latter was meaningless. They seemed to have stopped, basically, valuing education! A weekend away was out in the hills shooting animals... it was all a bit 'hill billy'.
Be aware that you need to be very careful about burning financial bridges in the UK. You'd possibly never be able to buy back into the UK market.
There's a reason why 20% of the NZ population don't choose to actually live in NZ!
Australia has a far bigger population and much more opportunity. State education can be patchy (whatever Australian say!) and also sports driven but there is perhaps more room for the academic. The economy is also in good shape.