More than 600 warrants of fitness were revoked by Waka Kotahi in Auckland after an investigation into an auto repair shop.
The transport agency has notified the Motor Trade Association - MTA - that 677 warrants issued over the last 10 months by one of its members, Newmarket Auto Repairs, have been void due to the lack of a certified WOF inspector.
Last week, Waka Kotahi wrote to the affected vehicle owners to inform them that their cars would need to be re-tested.
MTA chief executive Lee Marshall said Newmarket Auto Repairs' membership has been immediately suspended while a full investigation is carried out into the serious breach.
"For more than 100 years MTA has stood for integrity, safety and professionalism. We take that very seriously," he said.
"To say we are extremely disappointed would be an understatement.
"Our first concern is for people who have been disadvantaged and inconvenienced."
Marshall said the association was taking action to ensure customers were not out of pocket.
"Although responsibility lies with Newmarket Auto Repairs, MTA will issue, on an ex gratia basis, a $50 MTA voucher to all affected customers who are unsuccessful in getting reimbursement directly from the business.
"Fifty dollars is the amount Newmarket Auto Repairs advertised for a warrant. Customers can use the voucher at any MTA member business."
There would unfortunately, always be those that let everyone down, Marshall said.
"But we want to assure motorists that the blue MTA badge still stands for honesty, quality and professionalism."
The warrant of fitness played a vital role in road safety, and its integrity was not negotiable, he said.
"MTA is extremely disappointed that one of [its] members has broken our rules and Code of Ethics, and misled and inconvenienced so many vehicle owners."
Marshall said MTA had a set of high standards to which it held members to account.
"There is the opportunity for us to review and improve our member standards as a consequence."
Newmarket Auto Repairs have been approached for comment.
If you are someone who has had your Newmarket Auto Repairs WOF cancelled and would like to talk to RNZ about it, please get in touch via news@rnz.co.nz.