T-shirts and shorts are what I would recommend plus a medium weight pair of pants and a jacket. The Nth Island of NZ has similar weather to Australia, so averages 22-26C and the Sth Island averages about 20-25C. I was last in Milford Sound in Nov08, and it was 15C and WINDY (the winds come up from Antarctica), so you will need something that is wind proof. It will also get quite cool at night on the Sth Island. Take you swimmers, a pair of shoes that can get wet and good walking shoes. If you’re buyinjg new walking shoes, make sure you wear them at home first, or you will die when you get going.
The GBP buys 2NZD, so if you leave anything at home, you can always buy it there.
Also, take a lot of sunscreen, or buy some as soon as you get there. The Auckland City YHA is centrally located and costs about 25NZD (12GBP) per night in a dorm room. You could pay for a double room and not share and this is about 80NZD (40GBP) for a night.
I used to live in NZ, so if you have any other questions, let me know.