my suggestion...
When you arrive in Auckland International airport and get through customs make an immediate left in the lobby where peeps wait for peeps and you will see a big wall of hostels pictures, phone numbers and prices. Call Fat Camel Hostel in Auckland and reserve a bed, it will run you about 18 NZD for the night. Go outside and wait for the backpacker bus that runs about every half hour and takes you into Auckland, trust me its the cheapest way there. AKL is a good 35 to 45 minute drive from the actual city so 10 NZD is pretty damn cheap as far as Im concerned, the cab will run you between 30 and 40. Once you arrive at Fat Camel you will realize that you are right smack in the middle of the city close to the harbour, Americas cup village, sky tower, internet cafes, cheap ass shops, kickin fish and chips, curry like you wouldnt believe and best of all the Kiwi Experience can be booked just around the corner and I must be blunt, I highly recommend you use them
The SHEEPDOG pass is by far my favorite, its about 600 NZD but goes for like 16 days, hits the best of both islands, bungee jumping and all kinds of cool shit.
your gonna love it