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When I lived in Auckland I stayed in this pretty cool place called the Waldorf Apartments (there are two the cheap but equally good one is not on the waterfront but the waterfront is nothing special). Pretty sure for a month-long stay you could get a nice studio appartment for $300NZ a week which would be cheaper than hostels probably. For one or two weeks it would be closer to $400NZ I think. Only place I would definitely steer clear of is the Auckland Central Backpackers. That place is awful.
In Christchurch there is a pretty nice place called Drifters. When I was there they offered like a kind of weekly deal in a seperate house at $110NZ/week per person. Its far from exotic but great value for money.
Hope that helps. If anyone wants any advice on places to see while you are there I am happy to point out some of the good spots and have been through practically all of the South Island (not in ski season though).