If you are booking through the NZ website, use the booking engine on the front page. Just avoid the cheapest category and everything else earns miles. This works well with simple itineraries.

If you are trying to book through multi-city option there is no way to be sure to get the lowest fare that is mileage earning. Unrestricted is almost full fare and lowest fare could bring up non-mileage earning. In these cases I either phone to book, or use another site that is more flexible.