Certification for drawbars and drawbeams on about 2000 trucks and trailers has been revoked by the New Zealand Transport Agency (NZTA) as part of investigations into certifier Patrick Chu.
Of those, about 1700 vehicles will remain on the road but with restrictions on the loads they can carry. The exemptions will last for up to 12 months, when the vehicles will need to be recertified.
Drawbars and drawbeams are a metal coupling which links the vehicle to its trailer.
The latest safety alert issued by NZTA follows it announcing 1400 vehicles with tow bars inspected by Mr Chu had their safety certifications removed last year.
Steve Haszard, the managing partner of Meredith Connell, is leading the NZTA review. He said the exemptions offered to most vehicles meant business could continue.
"It's important we continue to ensure safety comes first, but we're also aware of the impact these decisions have on the heavy vehicle industry and heavy vehicle specialist certifiers.
"We've carefully considered this decision and feel that a one-size-fits-all approach is not applicable.
"The result of these exemptions means only around 300 vehicles require immediate recertification of drawbars and drawbeams. This will help offset the impact to industry."
Nick Leggett, the chief executive of the Road Transport Forum, said drivers and business owners would still feel the force of the restrictions.
He said most trucks left on the road would only be allowed to operate at 75 percent load, which would have a large effect on their work.
"Today's announcement that the situation with the Patrick Chu-designed drawbars and draw beams is worse than previously thought will come as a significant blow.
"The reduction in operating mass will have significant financial ramifications for many operators."
Mr Leggett said it would be a game of wait and see over what kind of impact it had on businesses.
"There is no doubt that the necessary load reductions … will prove very stressful for operators already dealing with extremely tight margins."
Mr Leggett said operators were entitled to assurance of quality when seeking a professional engineering service.
"For NZTA to not have appropriate oversight of these engineering services is pretty unforgivable."
The NZTA said it will cover some costs of recertification.