Here's the website for Origin Pacific, one of the main carriers (I think) which fly within NZ.
I have flown with them (Palmerston North to Auckland) and the only complaint I have is that they ran out of orange juice by the time they got to me so I had to have mineral water.
I think you can even book flights from outside of NZ on this website too, and reasonable fares.
http://www.originpacific.co.nz/Home
Diny