The owners of vehicles that may have been given a dodgy warrant of fitness have been given an extra two months to get them rechecked.
Nearly 25,000 people were contacted to get their vehicles rechecked after more than a dozen inspectors or garages were suspended for issuing dodgy warrants last year.
But only 16 percent have had a recheck so far, the Transport Agency's Steve Haszard said.
"We'd very much like that to be higher, which is why we're still urging owners to make use of the voucher system that we've sent out to them to go and get their vehicles checked."
By early January, almost 60 percent of rechecked cars had failed their first re-inspection.
Vehicle owners are responsible for ensuring their vehicle is safe, and their warrant is up to standard every day it is on the road, Mr Haszard said.
The agency is covering the cost of re-inspection, and owners now have until the end of March to get their vehicles rechecked.
The road safety investigation began when structural cracks were found in truck and trailer units in February last year.
It widened when it was found vehicle inspection company VTNZ had been using an inferior brake test, and NZTA had not been properly checking the companies that certify vehicles for the last two years.