There are fears new electric delivery vehicles could cause mental and physical stress on postal workers.
The fleet of imported eco-vehicles is costing $15 million, but John Maynard from the Postal Workers Union says trials of an early model weren't encouraging.
"The posties found it quite difficult steering in and out to all the letterboxes, so the first hazard we identified is that people are going to get sore shoulders and sore elbows from the heavier steering," he says.
"They've tested an electrical vehicle for five hours of people driving them, but they're planning on falling on half our shifts with eight or more hours of people driving them."
But E tū communications industry coordinator Joe Gallagher says while some issues did come up during testing, the feedback has been factored into the second generation of the vehicles.
Mr Maynard also says NZ Post has failed to tell posties they'll be expected to deliver parcels weighing up to 25 kilograms, as the company reduces the size of its courier van fleet.
That’s one of the issues that will be addressed, says Mr Gallagher.
"Issues about remuneration, health and safety, and hours of work all have to be worked through in proper discussions in the coming months," he says.
The new vehicles carry loads of up to 200kg but are still small enough to be used on the footpath.
The first shipment of 50 Norwegian-manufactured Paxster AS vehicles is due to arrive in mid-2016. The first battery-powered vehicles will be rolled out in Auckland.
There has already been a pilot of parcel and mail delivery in Taranaki, as well as road trials in Auckland and Wellington.
Vehicles will be used where permission has been granted by local council.