Three hundred heavy vehicles are banned from towing after the NZTA has revoked certificates for two key parts.

Before they can tow again, the drawbar and drawbeam need re-certifying.

It's after an NZTA investigation into ex-heavy vehicle specialist certifier, Patrick Chu, of Transport and Structure limited.

A team of senior industry engineers and experts at the Transport Agency undertook an investigation into Mr Chu’s files and discovered certification were issued for a range of non-compliant drawbars and drawbeams.

A further 1700 certifications have been revoked, but owners are can apply for an exemption, or will have had one automatically applied.

Nick Leggett is the Chief Executive of the Road Transport Forum. He told Mike Yardley it's likely NZTA hasn't had systems in place to ensure work standards were up with the law.

"It really is evidence of a regulatory failure. That means that any work carried out by certified engineers like Mr Chu has to be supervised.

"Unfortunately the regulator in this situation, the NZTA, hasn't had the oversight needed to ensure the work was at a standard which met the law. Not only will those in the road freight sector suffer, but the whole New Zealand economy will be affected by this."

LISTEN TO THE AUDIO ABOVE AS NICK LEGGETT SPEAKS TO MIKE YARDLEY