You should not have to pay customs duty at all.
Immigrants and New Zealand Residents Returning to Resume Permanent Residence After 21 Months or More Overseas
You are entitled to the following additional concessions and they apply whether the goods accompany you or are sent separately.
Household or Other Related Effects
Your household or other related effects (excluding motor vehicles, boats and aircraft) will be admitted free of duty and exempt GST for a reasonable time after you arrive, provided you can meet the following requirements:
you are coming to New Zealand with the intention and legal authority to take up permanent residence; and
you have lived outside New Zealand for the whole of the 21 months before the date of your arrival in New Zealand; and
you have owned and used the goods before the date of your departure for New Zealand; and
the goods are for your own personal use and not intended for any other person or persons or for gift, sale or exchange
goods of a commercial nature such as factory plant and office equipment do not qualify as household effects.
Note: Goods shipped directly after purchase to avoid local taxes in the country of export, and replacement electrical equipment operating to New Zealand standards, will not qualify for entry under this concession unless the importer can establish that the items have had personal use prior to their arrival in New Zealand.
You may have to pay fees if MAF decide that some of your possessions need to be inspected - but you will have seen the threads elsewhere on the site about this issue. This is something you need to watch out for, as some shippers or their NZ agents have been in the habit of scamming immigrants by charging FAR more under this heading than the official government department does. (If you haven't seen about this, it's worth doing a search so you are forewarned, and lay down some conditions when making your booking.)