When used on roads, paths and in public access areas, the legal limit for a motor in NZ for a "power-assisted cycle" is a maximum power output not exceeding 300W. Power-assisted cycles follow the same regulations as regular bicycles - all you need is a helmet! Anything exceeding 300W in power output is legally classed as a moped and so must be registered and ridden by a licensed driver etc. You can read more about this here.
There are no limits for offroad use.