KEEPING NATIVE FISH IN AN OUTDOOR SETTING
Some of the best New Zealand fish species for a native pond are probably common bully (Gobiomorphus cotidianus) and inanga (Galaxias maculatus). Inanga are one of the best, native pond fishes as they are active near the water's surface and can be readily trained to take dry foods.
Other species that may be kept in ponds include banded kokopu (Galaxias fasciatus), crans bully (Gobiomorphus basalis), Upland bully (Gobiomorphus breviceps) and giant bully (Gobiomorphus gobiodes).
Both species of eels (longfin eel Anguilla dieffenbachii and shortfin eel Anguilla australis) can also be kept; providing of course that they want to be kept!
Sourcing galaxiids and diadromous bullies and eels is best done during the whitebait season (spring) either by catching them yourself, or by trying to persuade a whitebaiter to part with a few for you 
. This way collection is entirely legal, the impact on the wild population is negligible (you are in fact saving the fish) and you have young stock which will both live longer and can be trained to their new environment and foods much easier.
Problems to look at with ponds and native freshwater life are mainly predation through birds and maintaining good water quality. Make sure there are overhangs or dense vegetation near the water's surface on at least one edge of the pond for fish to hide from birds under. Also make sure it is deep enough to be inaccessible from wading herons (more than around 0.30 m).
As expected, water quality will have to be similar to that of their natural water courses, and this is most difficult when it come to temperature; even low 20's °C are beginning to stress these rather heat intolerant native fishes. Once again, shade providing cover will help here; reducing the amount of sun hitting the water's surface. Deep water will also stay cooler, and a recirculating system will help keep oxygen levels up when water temperatures are high over summer (warmer water has less oxygen in it) and the fish are under heat stress.