http://flightbookings.airnewzealand.co.nz/vgrabview/
www.airnewzealand.co.nz if the above link doesn't work (grabaseat will be at the top)
The flights are updated every day.
I booked a flight from Wellington to Napier for $39....and because the return was the regular rate I did a bus trip on the way back for $20.
I've also booked one from Wellington to Kaitaia for $79 in a few weeks.
Basically these are flights that they haven't sold a lot of seats yet, so they offer them at really cheap rates. A lot of times they are odd days (like Tuesdays and Thursdays), but with a little bit of planning, you can make it happen. Some weekends/Fridays are also available.
The seats go pretty quick sometimes, and are only on there for a day (if they weren't grabbed), so you have to act quickly.