More than 1400 vehicles with tow bars inspected by Auckland-based certifier Patrick Chu have had their safety certifications removed.
The Transport Agency said Mr Chu was using the wrong assessment standard when he checked the vehicles, which means the tow-bars were incorrectly deemed safe.
The vehicles with them included more than 1400 trucks, motorhomes and vans.
Agency managing partner Steve Haszard said there was a risk that trailers could detach from the vehicles.
Mr Haszard has urged anyone who had their tow-bar checked by Patrick Chu to have it reassessed by another certifier.