She said car owners would become very reliant on thorough checks being done by those who issued warrants.
"There are a lot of dodgy guys out there who don't do good checks and now we are supposed to rely on their checks for a year."
Vehicles built before 2000 will continue to have six-monthly inspections.
Whangarei RoadSafety campaigner Gillian Archer hoped the new regulations would not see a reduction in safety.
She said the onus would be on vehicle owners and drivers to do their own regular checks, particularly of brakes and tyres.
Ms Archer said warrants were just a snapshot in time and drivers had a continuous obligation to check their vehicle to ensure it was safe.
She said some cars closer to the cut off point of 2000 were nearly 14 years-old and in vehicle technology were fairly old.
"The key thing is to check treads on tyres. They may pass at the time of a check but six or seven months down the track they may become unsafe."
Those mechanics who passed cars later than 2000 would have to issue drivers with clear instructions and warnings to check vehicles before it was due for another warrant.
The consequences of not having a vehicle up to warrant standards can be tragic.
A number of fatal accidents nationally have involved non-warranted vehicles.
Yesterday's changes means about 1.2 million cars, made after 2000, that were once subject to six-monthly WoF inspections will now be checked annually.
Northland road policing manager Inspector Murray Hodson said the most common causes of serious crashes in the region were alcohol, speed, high risk drivers and dangerous driving.
However, "smooth and worn tyres, cracked windscreens, rusty vehicles and other vehicle defects" were sometimes factors in vehicle crashes.
"The warrant of fitness check is a minimum safety check and shouldn't be confused with a vehicle maintenance check, Mr Hodson said.
"You don't have to wait until your warrant of fitness is due."
