it is quite easy for a zoo to get a permit to keep koi carp although I haven't seen them in a zoo myself for many years. They are really common in some places in the wild though so not hard to get hold of.
I saw a large pond/small lake in Auckland somewhere a couple of years ago (I can't for the life of me remember exactly where!) which was chock-a-block with koi. The surface was almost boiling in places with the fish at the surface. Most were dark-coloured but there were other colours in there as well.Are 'wild' Koi multi-coloured like the ones in Aquatic culture. i.e. do they retain their bright colours or somehow revert to duller ones? I imagine natural selection would weed out the brightest colours- but maybe there are no Predators?
Is it still illegal to keep things like Hamsters& Gerbils as pets in N.Z? I remember Mice and maybe Rats were okay, also Rabbits and GuineaPigs/Cavies, but not much else in that line.