Further to my earlier post, I emailed NZ Customs again. They have just sent me the following:

The only people who may import there used household and personal effects without the payment of tax duty and GST are people immigrating to New Zealand for the first time to take up permanent residence who have a permanent residence stamp in there passport and the legal authority to take up permanent residence or who have a work permit or study permit for over 12 months.

If you do not have this then the Client Service Section Auckland asks that you have an application lodged with New Zealand Immigration Service stating that permanent residence has been applied for and will be granted within 6 months at this they usually take a deposit or bond on goods usually 10% of the value which is refunded when permanent residence is obtained if other then Auckland then check with Client Services at that Port you are going to addresses are on our website www.customs.govt.nz under contact us.

You have 5 years from the date of the permanent residence stamp in your passport to bring in your used personal and household effects.

The last line says it all - 5 years! I am now running a special prize contest for the first person to find that snippit on the NZ Customs website!

ps. The text is a cut and paste from the email - their (there) spellings not mine....