bartbridge...
we didn't do campervan. There were times I wish I had but on balance we got some excellent accomodation, mcuh better than US or Australia.
We stayed in motels and hotels in big cities i.e. Wellington, Auckland.
Actually stayed in rented serviced apartments in Auckland.
There is a very good book you can pick up at some rental car hires or else at tourist Information Sites. It is AA Accomodation Guide, big book with yellow cover. Borders or other bookshops try and sell it to for $10/$12 but you can get it for free at above.
There is North Island edition, South Island edition and both Islands edition.
It gives you details on almost all motels/hotels, caravan parks etc.
Gives good info on what they have as regards facilities, even little helpful maps of streets etc.
Get it it will safe you lot of headache.
We hired with Apex Car Rentals, cars were old, I mean 8/9 years old and they had guts drove out of them but they were little cheaper than big companies. Shop around, you are in off peak season.
I have to say the staff were very nice and helpful.
Check for accomodation in major centres before you arrive, August should not be too bad apart from ski centric areas e.g Queenstown, Mount Hutt etc. Use the book and phone around.
The motels were excellent, you can get nice one/two bed units with full kitchen, washing machine and dryer. Great for the old washing.
Note in winter, weather will be colder and wetter in some areas, so do you want to be in camper van?
Check for mountain road closures before you head off, check if need snow chains on higher routes, should be ok.
Also if doing both islands then you will need to book Interislander ferry.
Now allow flexibility on actual travel dates. You may want to spend less/longer on one island.
Also beware the strait between the two islands can get very windy weather which causes huge sea swells. So bad that the crew are actually lying on deck along with the passengers.
You will enjoy it, scenery is wonderful, mixture of Norway, Canada, US, Spain, Ireland, Scotland and a few other places.