A retiring mechanic says current Warrant of Fitness laws are allowing dangerous cars to remain on the roads and he fears people's lives are at risk.

But the NZ Transport Agency says road safety involves more than a warrant check and it is the car owner's responsibility to ensure their vehicle is safe all the time.

In 2014, the frequency of Warrant of Fitness (WoF) inspections changed so cars registered from January 2000 onwards would only need a WoF once a year.

Vehicles registered before 2000 require six-monthly WoFs.

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The NZTA website stated the change had saved motorists time and money while maintaining road safety.

However, some local mechanics fear vehicles are passing WoFs with the minimum requirements which will not last until the next inspection.

Bald tyres were their biggest concern and they said the law change had made the roads more dangerous.

John Currie, a mechanic of 50 years, has recently sold his business and is working part-time, doing warrant checks twice a week.

He was deeply concerned about the state of some cars on the road.

"This change of regulation ... is allowing people to drive around on bald tires and no brakes.

"I see it every day... I get cars in that shouldn't be on the road.

"People are in danger from those vehicles."

Currie believed only cars registered after 2010 should have year-long warrants.

"There are vehicles on the road that will pass a warrant of fitness... and you look at it and think 'this is rubbish, it should be at the scrap yard'.

"People aren't looking at their vehicles, they're not eyeballing their tyres once a week or checking to see if their brake lights are working."

Currie said he frequently saw tyres with worn-out tread and some with wires poking out of the rubber.

"They could at any moment blow-out, explode, potentially causing an accident."

Another mechanic with 30 years' experience, who on the condition of anonymity, said tyres should have a minimum of 4mm to get a WoF.

"You get a farmer who buys a brand new Ford Ranger, doesn't have to do a warrant for three years and they smash the hell out them.

"That car has wheel alignments that are out, tyres are worn out and he still doesn't need to get a warrant because the law says so. It's absolutely stupid. And the same for the cars that went from six months to a year."

The lowest tread allowed on tyres is 1.5mm and had to be passed.

"You say 'it's passed legally but I recommend replacing...' which is often dismissed as people think they are in the clear for another year."

He said he feared driving on the roads knowing what he does.

Another mechanic, who asked to remain anonymous, said the "she'll be right" attitude meant people were not checking their vehicles as they should.

"The mentality is 'I've got a warrant for 12 months so my car is good for 12 months'."

He said the warrant was a false sense of security for people.

Bay of Plenty road policing manager Brent Crowe said police were committed to keeping the roads safe and adopted a compliance approach to expired WoFs they come across.

He said when an expired WoF was detected, police had traditionally issued a fine.

Fines often acted as a barrier to drivers getting the vehicle warranted, he said, and police had shifted focus to help drivers get their vehicles up to standard.

"This is achieved by offering the driver 'compliance', meaning the fine may be waived if the vehicle gets a warrant of fitness within a certain period of time," Crowe said.

National Party leader and former Minister of Transport Simon Bridges was the minister who led the WoF change and told the Rotorua Daily Post he stood by it.

"Obviously safety is paramount but we saved New Zealanders a lot of money with no significant reduction in safety."

He said while there was a debate about these issues at the time, the evidence then was clear and he believed nothing had disproved it since.

A written statement from the NZTA said passing a WoF did not mean a vehicle was safe until the next inspection and everyone needed to make sure their vehicle was safe every day it was on the road.

"The increase in deaths and injuries on our roads in recent years is concerning, and there are a range of potential factors likely to have contributed to the recent increase in serious crashes, including a significant increase in overall vehicle kilometers traveled on New Zealand roads," they said.

Along with safe speeds, safe road use, and safe roads and roadsides, the safety of the vehicles played a key role in improved safety outcomes, they said.

Data from the MoT and NZTA Crash Analysis System showed the proportion of fatal and serious injury crashes where vehicle safety was noted as a contributing factor ranged between 3 and 5 per cent for the five years before the change.

This stayed at four years after, from 2014 to 2017.