I did it for 3 months about 2 years ago. We had lived on the south island for a year prior which enabled us to gloss over parts of it on our 3 month road trip, but I would still say 3 months is a good amount of time to do it. My advice would be:
- Rather than renting one of the typical vans (expensive), consider buying a beater and selling it when you're done. We bought a Subaru wagon for around $1500 and sold it easily for about the same when we were done. You could do the same with a van, but they are considerably more expensive so there is more risk. I realize this isn't for everyone, but it's definitely worth thinking about. Cheap camping (usually $6/night) is prevalent so we had no use for a sleeper van, and with high fuel costs we were often the envy of fellow travelers. They had big vans full to the **** with stuff, and we had a little wagon with curtains that we could sleep comfortably in with our belongings in the front seat. We bought a used tent and slept in it probably 90-95% of the time, but a car sleep was always a good option as well.
- Download and rely heavily on the NZ specific camping apps. They are simply amazing. There were 2 that we used every single day (Campermate was the better of the two). They show you where campsites are, where bathrooms are, where water is, where free wifi is, where everything is that you will need. They were also helpful in pointing out attractions and things we surely would have missed otherwise. Some of it was typical tourist trap stuff, but some of it was far better than we could have ever expected. Secret waterfalls, beaches, etc that we never would have found, and often we were the only people there.
- Use Trademe for everything you need. It's a used marketplace that is an awesome hybrid of Craigslist and Ebay. I miss this greatly.
- Check out Bookme.co.nz if you're into any kind of typical tourist 'adventures'. We got awesome deals on things like whitewater rafting, paddle-boarding, etc. My advice would be to avoid any kind of 'tour', but that's just a personal preference. We paid a fortune to do a cave tour in Waitomo and it was one of the lamest days of my life. There are plenty of amazing places where you can cave/etc for free without the hand-holding of a tour guide.
- We had a little camp stove for the first month or so and were determined to make our own food as much as possible. We abandoned this when we found out it was actually cheaper to live off of Subway ($6 subs), Dominos ($5 pizzas), and the thousands of awesome and surprisingly cheap fish & chip shops. These little hole in the wall shops are one of the things I miss most about New Zealand. They are everywhere, they are cheap, and 90% of the time they are awesome. Both of us could typically eat for about $5-$8 total, which was far cheaper than almost anything we could make ourselves (groceries are also wildly expensive). Subway became our primary source of vegetables, with supplementary additions from the grocery store.
- There are a couple of outdoor stores in NZ, but they are pretty expensive and the quality is suspect so try to bring what you think you'll need. We needed new sleeping bags and ended up ordering from the US. Shipping was more than the bags themselves, but it was still a better deal.
Just to give you a reference point, our total expenses for 3 months were right around $5000 per person (2 of us), with an average spend of $56/day/person.
That's all I can think of off the top of my head, but I may pop back in if I think of anything else.