Postal workers are in a spin over a speed-based performance evaluation system they say is costing them hundreds of dollars a year.

Photo: RNZ / Alexander Robertson

New Zealand Post's pay model measures a postie's work rate, and how fast they should travel, meaning they are paid according to how much time a round of deliveries should take.

If posties are fast, they are paid more. If they are slow, they earn less. Every payslip is different.

Postal Workers Union's northern secretary Michael Hunter took the case to the Employment Relations Authority, arguing certain routes or "rounds", were beset by delays where posties had to slow down between mailboxes.

Those delays could include large walls, hedges or fences, hidden driveways or uncontrolled intersections.

Those were not covered by the performance system, Mr Hunter said.

The problem roughly accounted for the loss of an hour's pay for every postie each week and was particularly bad in affluent suburbs of major metropolitan cities, he said.

New Zealand Post's system was changed in 2011 to compensate for rubbish collection days when streets are lined with wheelie bins, and particularly windy days in Wellington.

The company argued that "lost time" was already covered within its system, which it said was not based on posties riding "freely and briskly".

The system was created using simulated studies involving skilled, trained posties, and already took hazards into account, it said.

The Employment Relations Authority sided with NZ Post and rejected the need to change the system.

Mr Hunter said he was very disappointed by the decision and the union would decide whether to appeal at a meeting on Wednesday.