If you're British (which I assume you are
) you can extend a Visitor's Visa for another 3 months, bringing your stay up to 9 months, but then you must stay out of NZ for 9 months before trying to re-enter. In special circumstances, such as waiting for a PR application to finalise, you can apply to extend your visa up to a total visit of 12 months. If you stay for the 12 months, you then have to stay outside NZ for another 12 months.