Due to the lack of data from NZ Post, I've created this mysql database from the PDF on the NZ Post website.
All efforts have been made to ensure that there are no errors, but there are no guarantees, if you need 100% accurate data please buy NZ Post's PAF data.
Update: New Zealand Postcode Database version 2
Version 1:
nzpostcodes.sql.gz (467KB)
nzpostcodes.sql.bz2 (313KB)