The New Zealand Transport Agency has ordered owners of 802 heavy vehicles not to use their towbars.
The decision came after the NZTA revoked the safety certification of towbars issued by Peter Wastney Engineering (PWE).
Initially the NZTA had issued a safety notice to 1,800 heavy vehicle owners in February, requesting drivers check their towbars and drawbars for visible cracks, and have them checked by another certifier.
The notice indicated that after several recent incidents, independent reviews of drawbeams and drawbars certified by PWE identified that they were not adequately designed for the loads to which they had been certified.
However, further checks since February revealed that 61 of 62 towbars inspected were not fit for use, according to NZTA operational standards manager Craig Basher.
"Given these findings we are taking immediate action to revoke the certifications for all towbars certified by Peter Wastney Engineering Ltd in order to address the serious safety risks involved," Basher said.
"These towbars will need to be removed, replaced, or inspected and re-certified by an approved independent heavy vehicle specialist engineer. The vehicles may continue to operate on the road in the interim, provided the towbars are not used."
The majority of vehicles affected are located in the Nelson, Marlborough, Tasman and West Coast areas.
Affected owners will still be able to drive their vehicles, but will be unable to use their towbars.