Unsafe vehicles are on the road due to the NZ Transport Agency's failure to monitor the companies that certify vehicle safety.

Lawyers have been called in to review 850 cases of compliance after the New Zealand Transport Agency failed to properly check companies that certify vehicles as safe for the road.

Transport Minister Phil Twyford said he was disappointed that the NZTA had failed to carry out its regulatory responsibilities to the standard he expected.

Twyford said the clear implication was that there were unsafe vehicles on the roads.

The NZTA had failed in its duty to properly check the companies that certify vehicles as safe for the road, and other services.

"When problems with these companies were identified, there was often no follow-up," he said.

Law firm Meredith Connell had been brought in by the NZTA board to review 850 open compliance files.

About 152 files required urgent legal or investigative review and that work is expected to be completed by early November.

NZTA chief executive Fergus Gammie said: "It is clear that our approach has not been sufficiently robust to categorically ensure the highest levels of regulatory compliance.

"The NZTA has been too reliant on self-regulation and has not devoted enough attention or resource to ensuring compliance."