This information has been provided by the New Zealand Customs Service. A link is provided below
You can bring human ashes into New Zealand with you, but you must declare them. We recommend that you have a copy of the person’s death or cremation certificate with you.
If you send human ashes by post, you must declare on the postal declaration that the package contains human ashes. You must include a copy of the death or cremation certificate in the package.
For further information please click on the following link https://www.customs.govt.nz/personal/prohibited-and-restricted-items/