Well..
We spent there 1.5 weeks and continued to Australia, and felt that we didn't even scratch the surface. The next year we arrived to the country with the plan to depart when we've seen everything.
We ended up spending about three months there and we have an approved residency permit to move there more permanently
If we had managed to sold our house last year, we might be there right now.
Even after the three months and 6000+ photos there are places we managed to miss. I've done itineraries to five friends so far..
Check out
http://www.waterside.fi/NZ-South-Island-Book.pdf
http://www.waterside.fi/NZ-North-Island-Book.pdf
Calendarwise I'd target for Jan-Mar, NZ can be a bit chilly country and their heating heritage is very british. The spring may or may not be warm.
The best of NZ is outside of the bigger cities. Compared to the rest of the country, there's really very little to see in Christchurch, Wellington or Auckland.
NZ is all about experiences and the ozzie friend is right -- but it's not just marketing. It wasn't really just NZ but one could almost say that NZ changed what we want from travels.
For a short trip (anything less than a month) couple of thoughts;
Queenstown is the adventure capital of the world, if you don't want an adrenalin shock it's still a good base for plenty. Take a helicopter ride to Milford Sound and back, going across the NZ Alps is beautiful experience. The scenery around Queenstown is superb and plenty of wines to experience in the Otago region.
If you go to Christchurch, visit Akaroa. Part of France in that region. Great restaurants (C'est La Vie).
Fox Glacier or Franz Josef Glacier is great but usually off the route. And you really want to do a heli-hike or something similar (NZ has the most helicopters per capita
At the northern end of the south island, Nelson and the neighboring Abel Tasman National Park are great for beaches and hiking from beach to the next. There's a water taxi that goes between them. Kiwis are very outgoing people and the infrastructure for hiking and tramping is superb. Don't forget the WoW - World of Wearable Arts exhibition that also has an extensive collection of classic cars.
Next to Nelson is Blenheim, which is home to the famous Marlborough wine area. In addition to the wines, visit Peter Jackson's Omaka Aviation Heritage Center. Queen Charlotte track in Marlborough Sounds is amazing but again, needs more time and is all about being outdoors.
On North Island, Rotorua for the thermal activity. Forget the most advertised places and drive to Orakei Korako. BBC shoot a dinosaurs document there and it's the least touristic geothermal park.
From Rotorua you can do a day trip to White Island. Drive to Whakatane, board a helicopter to the island. Explore and return. You'll be visiting an active marine volcano. Absolutely a must.
Waitomo was another favourite with it's caves. One can choose either the "wheelchair accessible" caves, or the less accessible. We did the first while waiting for our scheduled second. And booked a few more of the less accessible
St. Benedict's Caverns is nice as it's a dry trip, Black Abyss and the full day Lost World are something that include everything.
In the northern part both Coromandel and Bay of Islands are beatiful coastal areas with islands, beaches, plenty of water activities, sailing, you name it.
NZ has excellent domestic flight network that make hopping around surprisingly easy.
After NZ Australia was pretty boring
Not that we didn't enjoy Australia as well.