Jetstar's on-time performance in the last 6 months has been better than Air NZ on the main trunk routes, but they fly far fewer flights so this can be looked at in two ways. One of Air NZ's issues in particular is at Wellington where screening at the gate becomes a bottleneck with the move from 737s to A320s with 40 more people. If people all turn up at the last minute it really slows boarding. This should hopefully get better when the airport moves to centralized screening at the end of the pier.
I've flown a lot of Air NZ WLG to NPL flights over the last 12 months - usually the last flight of the day on a Sunday evening. Because it's the last flight of the day, they often appear to deliberately delay it and wait for passengers off of connecting flights. I actually consider this a good thing.
**Edited for missing word