I'd second what Mr Keates said, leave the ski's in the uk! The skiing at Whakapapa is not too bad but there are a lot of rocks around - I definitely slightly damaged the skis I hired when I went off piste. Plus the season is not that long so you won't get that many opportunities to get down there. Last season started in June but the snow was crap until about the middle July. By the middle of September the snow was getting pretty thin again so its only about 8-10 weeks. The South Island resorts do have longer seasons but I highly doubt you will get too many opportunities to get down there.
What you can do is when you go to Whakapapa, buy a 5 day lift pass (can be used any 5 days in the season). Ski hire is about £9 per day I think.