AndrewTD: I think one of the major challenges with NZ using nuclear power is that, for resilience purposes, you would need two plants. Nuclear plants are, as I understand it, very very expensive to build. Whilst we probably only need the power that one (expensive) nuclear plant could generate, we would have to pay for two for security of supply. And/or continue to maintain an equivalent amount of other types of power generation.
There's also the downtime for maintenance that you have to factor in, plus as anyone who has ever played SimCity the last thing you want is your one power station for the country going offline ad then having to put one in quickly!