The Norwegian "Paxster" electric deliver vehicle will enable New Zealand postman to carry and distribute 200kg of parcels and mail at the same time.

New Zealand Post's new four-wheeler delivery buggies are putting posties and the public at risk, unions representing postal workers claim.

The state-owned enterprise sent a lawyer's letter to the Postal Workers Union of Aotearoa (PWUA) demanding it retracts criticisms it has made about the quad bike-like Paxster electric buggies which posties will have to use - replacing bikes and mopeds.

NZ Post said the criticisms made to media about the Paxsters were "misleading" and also an act of "bad faith" by the union, given the ongoing negotiations about the introduction of the Paxster.

SUPPLIED NZ Post will be "vigorously defending" itself around its Paxster purchase and employment dispute issues at mediation.

But PWUA spokesman John Maynard insisted the union allegations were accurate - among which are claims they could cause posties neck and back strains and that Paxsters are footpath hogs and it is only a matter of time before a pedestrian was hurt.

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NZ Post refused to comment on the union's claims, but its central and southern operations general manager Matt Riordan said it would be "defending [itself] vigorously" at upcoming mediation with the unions.

ROBERT CHARLES/FAIRFAX NZ Stability issues have been addressed and Paxster alternative, Kyburz remains popular with posties, John Maynard said.

​In April, the postal workers' union and E tu's combined 2100 postal workers are undergoing urgent mediation with the SOE over issues with the Norwegian made Paxsters.

New Zealand Post (NZ Post) is rolling-out 500 Paxsters, eventually costing $15 million.

Health and safety requirements around the buggies were not adequately addressed before NZ Post began to deploy the first Paxsters, Maynard said.

After being trialled in 2016, then adopted in 2017, some Paxster-driving posties have claimed shoulder and neck strains from the buggy's unassisted steering and heavy helmets and long shifts, he said.

Since Paxster adoption, posties have also switched from six day mail delivery cycles to four day cycles, often requiring nine hour plus shifts.

"Six months in, the union is concerned about an increase in the number of strains, sprains, incidents, near misses and fatigue related incidents," Maynard said.

Maynard said some steering issues had been addressed, but the union was "not satisfied" low-speed steering has improved enough.

Meanwhile, rear brake lights on the road-legal 45km capable Paxters won't come on unless the handbrake is applied, meaning motorists behind could get a nasty surprise, Maynard said.

The high-sided buggies also have a rear view blind spot of around four metres Maynard said. He believed that could endanger pedestrians with whom the Paxsters often share footpaths.

It's only a "matter of time" before someone got hurt by Paxsters, Maynard said.

The union said posties preferred a Swiss-made, Kyburz buggy.

However, Kyburz models were involved in several crashes, including one that injured a postie. Maynard insisted those stability issues were addressed.

E tu national industry organiser Joe Gallagher said "we want an outcome that works for everyone".