In that amount of time, you should decide whether you want to focus on the north island or the south island, because that is not much time! We flew into Auckland, spent a few days there to acclimate, and then flew to Christchurch on the south island. To me, one of the best things about NZ is that you can camp anywhere! This means much less planning is required (assuming you plan to camp or camper van, which is what we did). We just drove around, doing hikes everyday in gorgeous locations, and exploring towns and cities when we came to them. I think we were there for 3 weeks? We had a loose itinerary of what we wanted to see (2 day hike in Abel Tasman along the sea - highly recommended, you can see glow worms!), see a glacier, see penguins, visit Milford/Doubtful sound, go up into the mountains, hit up the wine countries, but other than that, we were very flexible. Also, we weren't interested in the adventure tourism of Queenstown.
So maybe do some research and get an idea of:
A) what island
B) what highlights you want to see
C) your accommodation plan - camping, camper van, hostels, B&Bs, airbnb, etc, and decide if the time you are going warrants booking things or playing it by ear (if it is not high season you may be fine to just find things)
D) craft yourself a loose itinerary that doesn't require too much driving each day and allows you to hit the major must sees but also allows you to explore as you go.