Depends on your budget really. I've met loads of people who have spent months in Australia and only a month in NZ and they have all been gutted that they haven't done it the other way around. NZ has so much to offer in such a compact space, it's an amazing place to travel. It would be possible to do it in 6 weeks but I would recommend 8. South Island perks for me were Kaikoura, Mount cook, The Catlins, Fiordland & The Sounds, Fox & Franz Josef Glacier and in the North Island, Taupo & the Tongariro crossing, Napier, Raglan, Waitomo, The Coromandel Peninsula, Bay of Islands & the Kauri forests - (which alot of people never visit and for me are one of the most amazing places).
NZ is set up for backpacking like no other country I've ever visited. Camping is abundant, you can camp for free at many places, DOC (Department Of Conservation) has the cheapest pay for camping but no showers generally, and there are loads of privately run campsites. Bear in mind that you will be arriving in winter and It will be bloody cold in many places in the South island, and not much warmer in the north. Depends how hardy you are. Personally I reckon if you have enough money to rent a camper you may as well stay in hostel dorms for$20-$25 per night and camp if you think its warm enough.
June / July would be perfect for skiing. Im not a skier but I know of Hamner springs, mount cook in the south. National park near Taupo in the north.
[ Edit: Edited on 30-Apr-2009, at 03:28 by eggman ]