In reply to craig1983:
Hiya - there's lots - here's a few company names to get you starter. The backpacker type vans you see around most often are from Jucy, Wicked and Escape. They're mostly for 2 people tho. Not sure if you'd fit 4 people in them tho. Jucy do bigger vans, as do pacific horizon and maui.
Be aware of the freedom camping act, that came in to place late 2011, which means local councils can decide if freedom camping (i.e. sleeping in a tent or vehicle without a self-contained toilet within 200 meters of a road) is allowed, allowed only in certain areas, or banned completely. They have fines and penalties in place. The campervan places are not very quick to point this out, and emphasise the freedom aspect instead.