I have done the same as you in 2007. You will be asked to pay GST and then to sign a DOU (Deed of Undertaking). I argued with the GST issue and they let me off. The also then let me off having to sign a DOU. I don't know if it was because the bike was older or not. You should check as some bikes are cheaper in NZ, others are not.

If you really want to do it right, buy a 5 year old bike in the UK and bring it over with you. I sold my old bike for 30% more than I paid for it 4 years ago. Secondhand bikes seem to hold there value very well out here. It would have been worth peanuts in the UK (being so old). Like me, you can then sell the UK secondhand bike to reduce the cost of your new one: Whic is exactly what I am doing.

Good luck.