A quick Google pointed me to this: https://www.nzpost.co.nz/personal/sending-within-nz/how-to-address-mail/postcodes/postcode-maps

And also this: https://www.nzpost.co.nz/personal/sending-within-nz/how-to-address-mail/postcodes/how-postcodes-work

To quote the second link:

Postcodes are four digits and each digit has a specific meaning.

The first digit of the postcode represents one of ten machine 'lines' (numbered 0 - 9), which process mail for a specific geographic region. These run north to south in ascending order.

The second and third digits reflect postal sort areas and our New Zealand Post delivery network - urban posties, New Zealand Post PO Boxes, Private Bags and RuralPost.

The fourth digit identifies the specific box lobby, rural delivery round or urban area.

Wikipedia says the first 2 digits relate to the region so 00-69 is North Island and 70-99 is South Island.

Source

Didn't expect it to be that easy. Thanks lads.