New Zealand Post is adopting the use of the four wheeled electric vehicles.
A union for posties is accusing New Zealand Post of arrogance in the wake of proposed changes to how and when mail is delivered.
John Maynard of Postal Workers Union of Aotearoa, which represents about half the country's posties, said there was a proposal for posties to work fewer days and longer hours but said NZ Post was not talking to the union about the move.
"The company is going around predetermined that the union will agree with the company to extend from six and a half hour days to nine and a half hour days," he said.
"And they're going ahead quite arrogantly without having consulted with the union.
"In fact they've said they won't be consulting with the union and that's a breach of the collective agreement."
Maynard said there was the possibility the longer hours, combined with the compulsory reasonable overtime, could lead to 11 hour days for posties.
But New Zealand Post service delivery chief operating officer Ashley Smout said they had discussed the changes with staff and received mainly positive feedback from the proposed changes to working hours.
"That would mean longer hours per day and that might not be for everybody but the consensus we've got so far is there's a huge amount of interest in fewer days and longer hours," he said.
He said the aim was for everyone to have at least two days off in a row, instead of the six days a week posties usually worked.
The disagreement comes at the same time as the mail deliverer introduces electric delivery vehicles that have not won overall favour with the posties who trialled them in New Plymouth and Upper Hutt last year.
In the New Plymouth trial of the three-wheeled, hi-tech vehicles there were five incidents in the first two months.
In one incident a postie sustained two fractured ribs after he lost control of the vehicle when rounding a corner.
Despite that record Maynard said posties preferred the three wheelers to the 500 four wheeled vehicles he said NZ Post planned on buying in the next two years. He said if, "and there's a big if", they did agree to the longer hours, they wouldn't have to work the overtime.
Maynard said posties were happy to drive the four wheeled vehicles for short periods but did not want to be in them eight hours a day because of the heavy steering.
"Posties are riding them for three to five hours at a time, and they say that's enough," he said.
He also cited health and safety concerns with the vehicles.
"If you're stopping and starting all the time...people are worried about getting an occupational overuse syndrome from repeatedly applying the handbrake," he said.
However Smout said there had been positive feedback from the trials of the new vehicles and people could take some time to get used to them.
"Everybody that has been on them doesn't want to go back to the old ways of working," he said.
"I'd say let's not judge too early, let's give people a chance to get used to them and then have those conversations and if there needs to be training, then we'll invest in that."
Choosing the four wheelers over the three wheelers was a technical decision around battery life.
"The longer we can have these vehicles out there on the road the better. So it was a pretty difficult call to make. We're very happy with that choice," he said.