Hi Pauline,
The customs duties you are charged depend on what you are importing and how it is classified. Different products have different rates of duty depending on what they are, where they come from and what they are made of. Like drena says, Customs will be able to tell you what the rate is.
The NZ Customs website is also useful:
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Check out what countries also have preferential rates - if you import from these countries the duties will be less.