A specialist engineer says he is "deeply disturbed" at how much effort the Transport Agency is putting into a clampdown on motorhomes and horse trucks.
Waka Kotahi now requires vehicles that have had cab modifications since 2005 to be signed off by specialist engineers.
A heavy vehicle certifier, who RNZ agreed not to name, said the agency was going too far.
"I am deeply disturbed by the amount of resource NZTA has put into this [in relation to actual safety issues] and I think the reasons for it should be communicated," he said.
Some cabs might be unsafe "but the criteria they are proposing does put the hoop too high".
"There will be kickback from plenty."
The agency said it was trying to balance safety risks with impact on owners.
However, the engineer said NZTA's new cab-approval guidelines for certifiers - in what is called a technical bulletin - appear "to make a lot of things impossible".
They did not take into account the range of vehicle types or if the original state of the cab was strong enough, he said.
Some cabs had the right certification, called an LT400, done before this clampdown by engineers employing "pragmatism and engineering judgement", he added.
Other engineers have told RNZ that Europe and US did not apply such standards to safety in these types of vehicles.
That has been seized on by a group combining the Motor Caravan Association with horse truck owners, that is demanding Waka Kotahi justify its clampdown.
"If New Zealand is going to be the only country in the world that requires a certification for a cab cut-out, then it needs to be based on substantiated evidence," group spokesperson, Grace Evers said.
She has imported horse trucks from Europe for years.
Evers questioned the rigour of tests on truck cabs done in 2018 by a Palmerston North engineer, which alerted NZTA to cab modifications, which the agency cites in its technical bulletin. The engineer would not comment to RNZ.
Coachwork Central said it was involved in the tests was now fixing cabs caught in the clampdown.
"Was this company experienced in crash testing and were they following internationally recognised methods to carry out such tests?" Evers asked.
The technical bulletin shows stripped-back truck cabs being tested.
A second certifying engineer said this was not the right type of testing.
Certifying engineers are often reluctant to identify themselves, saying it could impinge on their businesses which were authorised to operate by NZTA.
Engineers have been part of a committee set up by the agency recently to try to resolve the problem of retrospectively checking if cabs are up to scratch.
The agency acknowledged that "more collaboration" was needed.
Evers said the clampdown had caused major problems so broad consultation was required.
"If their [vehicles'] safety is in question, then it has to be looked into in great depth involving respected engineers across the industry," she said.
National Party associate transport spokesperson Barbara Kuriger said Waka Kotahi "has far better things to do" than enforcing rules on older vehicles, and on new vehicles already certified by manufacturers.
"By all means make it mandatory for new COFs (certificate of fitness) going forward but not on those already certified here or from overseas. Common sense needs to prevail."