A former top road policing officer says the changes to the warrant of fitness system will result in more deaths on the roads.
The Government has decided new cars registered after 2000 will need a yearly warrant, instead of a six-monthly check once they are six years old. The newest vehicles will not be tested for three years following an initial inspection.
Retired head of the Serious Crash Unit in Waitemata Stu Kearns attended more than 1000 fatalities in his 28-year career.
He said fatal crashes are still being caused by basic mechanical faults that would have been picked up in a warrant of fitness inspection.
"It may not happen immediately, but over a period of years I believe the good work the police have been doing to try and reduce the road toll to under 300 is going to be ruined. I think we'll see injuries and fatalities increase."
Mr Kearns said New Zealand drivers are not in the habit of getting their cars serviced in between warrants, and the police are unlikely to pick up on problems when they carry out roadside checks.
The Government said the changes will be in place by July next year and will save motorists, businesses and the Government $159 million a year.
But Mr Kearns said the social cost of a road fatality is about $3 million, so any savings will be lost over time.
The Police Association has welcomed extra Government money for road policing but says more frontline staff are needed to manage the new system.
Police funding will be increased by up to $5 million, but Police Association president Greg O'Connor said this does not mean extra staff because constable numbers have been capped.
Mr O'Connor said the vehicle compliance work will mean less time is spent on other frontline tasks.
The Transport Agency said the increased funding is likely to come from the National Land Transport Fund but that will not be assessed until April.