There are 2 or three backpackers guides available in NZ as well as YHA guides. Normally you only need to book up to a week ahead for backpackers. However if you ar looking for double rooms or family type rooms in backpackers, you best book earlier as those are quite scarce. The Lonely Planet guide to NZ is quite good. It gives you an idea of what is available.
Booking is less important with Backpacker style accommodation.
Besides the specific facilities outlined below, there are numerous motels and hotels of varying quality and price. They consequently tend to be cheaper than hotels. Cabins in motorcamps are usually the next step 'down' from motels. YHA are a few dollars more per night than Backpackers. Twin bed rooms, small shop and dining facilities on the premises, tourism information available, etc.