I went for 5 weeks last July/August and am going again this year at the same time - still so much that I haven't seen. There is really so much to do and see I don't really know where to start so I won't be specific.
I hired a car from Auckland to do the Bay of Islands then flew to Christchurch. From there I did the TranzAlpine train to Greymouth before joining the very good Magic Bus (felt very welcome even though I'm 55 and most of the other were 20s - 30s).. Then travelled back to Auckland via Franz Joseph, Queenstown, Milford, Dunedin, Lake Tekapo, Christchurch, Kaikoura, Nelson, Abel Tasmam, Picton, Wellington, Napier, Rotorua and Mt Maunganui.
Stayed in backpacker hostels all the time and was never disappointed.
I bought the Rough Guide and it was excellent so I've just bought the new edition. Other folk that I met swore by Lonely Planet but the new one has had some poor reviews on Amazon. Whichever you buy make sure it is as up to date as possible.
Could go on forever but please post again if you have any specific questions or post an email addy.
You will love it. Good luck.