Contact the ombudsman if you get no luck: https://www.govt.nz/browse/engaging-with-government/complain-about-a-government-department/complain-to-the-ombudsman/
They can help with any issues relating to government including crown entities and SOEs (they even specifically point out NZ Post). No department wants to get a call from the ombudsman so they may come to the party with regards to fixing it once and for all.