The performance of the under-fire NZ Transport Agency's regulatory functions will be reviewed following the emergence of more concerns, Transport Minister Phil Twyford has announced.

Car passenger William Ball died in January after a frayed seatbelt failed in a vehicle given a Warrant of Fitness by a Dargaville company.

The NZTA admitted knowing since 2011 that Dargaville Diesel Specialists was failing to carry out critical WOF checks but, in line with its approach at the time, was more educator than enforcer.

The owners of nearly 2000 vehicles that received warrants from the company were given offers to have them rechecked.

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Transport Minister Phil Twyford announced in October that Law firm Meredith Connell had been brought in by the NZTA board to review 850 open compliance files and taken over its compliance function because it had failed to carry out its regulatory responsibilities to the standard he expected.

"Since then, given the information that I, and the public, became aware of this week around the agency's performance, it is appropriate to ask for external advice on NZTA's regulatory performance," Twyford said today.

"I have formally instructed the Ministry of Transport to review NZTA's regulatory performance. There are a number of issues that need to be addressed to provide assurance to me, and the public that deficiencies around NZTA's regulatory performance are identified and addressed, and that is why we are asking for advice on what changes to the regulatory function are required."

The review is due to be completed by the end of March next year.

NZTA board chairman Michael Stiassny told RNZ yesterday Ball's death was a "horrific event" but warned the Dargaville case would not be the end of revelations about the agency's regulatory failings.

"We've made it abundantly clear there will be more," he said.