Best campsite in Auckland is the Kiwi Camp at Takapuna Beach. Head north across the Harbour Bridge and turn right. It's right bang on the water's edge and you share the same view as the $12 million mansion next door. Buy a Kiwi Camp card for $30 and you'll get 10% off. They'll also give you a map with directions to all their other camps.
The other big name in campsites is Top Ten. They also do a club card for $30 - 10% off. Top Ten camps are a bit more expensive than Kiwi, but they're usually a bit newer.
Head north from Auckland to Cape Reinga. You'll be there and back in three or four days - depends on how much driving you can handle. Must Do stops on the way include the Kauri Museum at Matakohe (Take SH1 north, turn left onto SH12 at Brynderwyn). Ninety Mile Beach (SH12) and the Kauri Forest at Waipoua. Take the car ferry ($12) at Rawene and keep going north. Don't be put off by the gravel road to Cape Reinga - campervans are all-wheel-drive-off-road-under-water-go-anywhere machines!
Come back south on the east coast (SH 10) to check out Waitangi, Paihia and Kerikeri. Top Ten camps at Kerikeri and Whangerai are good.
If you don't want to go that far north, hit the thermal pools at Waiwera (SH1 north from Auckland). Campsite round the corner (KiwiCamp I think). We'll want to see the photos of you coming head first down the big slide though!
If you head south through the Coromandel check out Miranda Springs (campsite and pools side by side about 15kms west of Thames) and the Driving Creek railway on the outskirts of Coromandel town. We got to the top of the Coro Peninsula in our van, but we got stuck and had to be rescued
(What was I saying earlier!?) The drive from Coro Town to Whitianga is awesome. Keep rolling to Hot Water Beach, but get there at low tide to enjoy the Hot Water part of the beach.
Keep going east along the Bay of Plenty. The campsite at the bottom of Mount Maunganui is even better than Takapuna. Then on to the Thermal Wonderland at Rotorua and on through Taupo, where it will rain. A lot.
You'll have to make a choice then - do you want to see Napier and Hawkes Bay or head down to Welly and come across to the Mainland?
Once over the Cooke Straight, you can go clockwise on the South Island (Picton, Kaikoura, Christchurch etc) or anti-clockwise (Picton, Nelson, Greymouth etc). West Coast for the glaciers and Milford Sound, East Coast for whale-watching. Shanty Town just outside Greymouth is a good day out for the kids.
Lake Wanaka is fantastic, but will probably be busy with skiers. The best campsite there is Mount Aspiring campervan park - keep going south along the lake and its on the left about 1km out of town. Take the Cordona Pass to Queenstown for breath-taking views.
The campsites in Queenstown will be heaving at this time of year, so you might want to push on to Te Anau. Use the Kiwi camp there - the Top Ten is next to the lake, but they put two vans on each pitch, so you'll hear the neighbours snoring. Watch out for the sandflies..... You'll need a full day to get to Milford, do the boat trip and get back to Te Anau. We booked when we got to Milford and they let the kids go free. Do the Nature Experience and get soaked under the waterfalls.
Hit Dunedin for the Albatross colony and MacFods cafe. The fledgling chicks might still be in their nests. Invercargill for Burt Munro and........ Burt Munro. The campsite at the agricultural showground in Invercargill is, well, an experience.
There are three decent campsites in Christchurch. Amber Park on Blenheim Road, Top Ten in Papanui and North South just out near the airport. Top Ten is pricey and noisy. So noisy that we got up and went to a different camp at 3.00am. Amber Park is on a main road, so the hoon's might keep you awake. The facilities at North South are a bit older, but it's nice and quiet (once the planes stop for the night) the people who own it are lovely.
Hope that helps.