There is a list of animal collections stickied at the top of the New Zealand forum, which is arranged in two ways - firstly north to south; and secondly by category (e.g. aquarium, native collection, etc).
Zoos of New Zealand
There are only two aquariums which could be considered under "most impressive" - and by North American standards neither is "impressive". Kelly Tarltons in Auckland is the larger of the two, but the National Aquarium in Napier is my favourite of the two. If you are used to "mega-aquariums" then you'd be disappointed in both, but if you are more interested in seeing NZ fish than in the size of the facility, then both are good.
The largest collection of natives is at Auckland Zoo in the section called Te Wao Nui.