Backpacker Board is the best forum I have found for Backpacking in New Zealand. It is fairly big and packed with travellers that have either been there an done that, are out there right now or locals who are involved in the backpacker business. Its a great place to look for travel companions and to get advice on everything.
They also have a range of original articles and a notice board which is good for picking up cheap cars, lifts and flats.
Start by visiting the forums.