That is because this product would cost over $300USD or more to ship to NZ. Most suppliers will not ship a product if it costs that much. Also there is the issue of the power cord being a 110 . Im not sure it would be compatable.
Have you tried calling gateway for that model without a warranty. They sell many items that have no warranty for much less then they sell them brand new. They are tested and recertified and then heavily discounted. And they may ship to NZ