Spent 10 weeks in New Zealand Jan-Mar 2008, excluding airfares we (2) lived on €550 per week! That was not a budget, it was just what we spent. All food, car hire, petrol, overnight accommodation etc included. We drove 3,500 KM on the trip, staring in Christchurch, down south including Steward Island, up the West Coast, then a mad dash up the North Isalnd for 2 weeks.
We stayed in the huts/cabins in camp sites which are great for meeting people. We did several of the "Great Walks" and so stayed in DOC huts. Food was bought about once a week in big supermarkets. We often ate a lunch on the road and had a light dinner. Not too many opportunities for fancy eating out, but we had a few.
It is a totally outdoor environment, so unless you spend lots of money bungy jumping etc, it is relatively easy to survive on little, which was not our objective, especially seeing the distance travelled to get there.
Our cabins/huts/apartments ranged from €10 per night, for both of us, up to a very expensive €60 for a lovely studio apartment on a South Pacific beach. get hold of the Jasons or AA guides when you arrive or join one of the Camping organisations (Top-10, Kiwi) for lists of places to stay. We relied on the Lonely Planet guide and were only let down once in the quality of the accomodation.
Going back in January 2010 and will probably up the accommodation quality a bit, maybe by €5-10 per night. Will certainly up the quality of the car, 10 year old Nissan Sunny had a hard job on some of the mountains and dirt roads. So another €5 per day for the car next year. Considered a camper van, but the flexibilty of a car and the opportunity of meeting people in kitchens etc in camp sites outweighs the van.