Postal workers say their state-owned employer needs to be investigated after a report it took 15 days for letters to be delivered.
An Investigation by the Sunday Star Times found it took that long for some letters sent by reporters from Hawera in Taranaki to be delivered to some locations in New Zealand.
The Postal Workers Union says its staff can be charged with misconduct under a clause in their collective agreement but it is NZ Post itself that is guilty of serious misconduct.
The union says it will be writing this week to the shareholding ministers, the minister of finance and the minister of state-owned enterprises, with information about serious misconduct.
Posties in some branches are being told to give priority to the delivery of parcels and to bring back undelivered letters at the end of their working day, the union says.
It also has issues with new electric delivery vehicles.
The union says it recognises the steady decline in letter volumes with the change from traditional letter mail to electronic communication.
"However it is one thing for NZ Post to adjust to declining mail volumes, it is quite another for the company to act in a manner which will ensure mail volumes decline even further."
As mail volumes declined the state-owned enterprise moved from next day across town delivery to three days, removed roadside postal boxes and changed targets for delivery of Fastpost mail.
"Just as a charge of serious misconduct against a postie would be investigated by the management, the union believes the charge of misconduct against the company should be investigated at the highest level - by the state owned enterprises' shareholding ministers who represent the interests of the general public."
NZ Post should abandon its price increases due on July 1 until the company can regain the confidence of the public, the union says.