VTNZ have acknowledged a significant breach at its Masterton branch after buses, including school buses, were certified by an unqualified inspector.
Emergency safety checks have now cleared the 11 buses in Masterton that had been operating with invalid certificates for months.
Brian Sara, operational excellence manager for Vehicle Testing New Zealand (VTNZ), said an anonymous email tipped off the company in July that an unauthorised inspector had certified 11 buses over the preceding four months.
"It was horrendous to think any unsafe vehicle was out there carrying passengers, some of them schoolchildren. Those buses were immediately recalled and checked by our head inspector," Mr Sara said.
"Putting children at risk was the last thing we wanted."
Masterton-based company Tranzit Coachlines owned 10 of the buses, which ranged in size from smaller vehicles to a large tour bus. The other vehicle was owned by Rideshop.
The recertification checks did not find any fault with any of the suspect vehicles, Mr Sara said, which had been the expectation.
"There was no safety risk whatsoever but initially we couldn't be certain so we have to move very quickly."
The New Zealand Transport Agency audited the station after the recertifications were complete and "a formal process" was conducted with the unauthorised inspector.
"He was told in no uncertain terms that this could not happen again".
Mr Sara said the inspector was experienced and qualified to check cars and heavy trucks and had been training "hip-to-hip" with an authorised colleague immediately prior to the breaches.
That authorised inspector had retired in March, Mr Sara said, and the invalid checks were conducted when the only other authorised inspector in the region was absent.
The unqualified inspector had used his retired colleague's identification to enter bus certificates he issued into the Landata system, Mr Sara said.
The inspector is not yet authorised to check buses but has completed his training and is waiting for a final New Zealand Transport Agency test that would qualify him, Mr Sara said.
VTNZ has 86 stations and 600 inspectors nationwide, he said, and the series of breaches in Masterton were unique and the most significant to date.
"We get the odd incident, the odd guy crossing the line but this many vehicles is certainly not something we were expecting. It just multiplies the risk."
Kate Styles, NZTA regional manager of access and use group, said that while no safety issues were found with any of the buses involved, the agency "takes breaches of the vehicle compliance process very seriously". She added: "It is important that the public has confidence in the integrity and safety of this system, and we're very disappointed that the process was not followed correctly with the inspections of these buses."
Ms Styles said VTNZ had completed an internal investigation and the agency was satisfied "appropriate steps have been taken to ensure that this doesn't happen again".
Keven Snellgrove, Tranzit Coachlines transport manager, said the company had remained confident that each of their buses were safe and fit to operate.
He said: "All of the vehicles passed the (re-certification) checks without a hiccup."
Tranzit Coachlines was founded in Masterton and is today one of the largest family-owned national passenger carriers in New Zealand.
David Lee, director of Rideshop, said he also had been certain their vehicle had been safe to operate.
"All of our vehicles are completely fine and I believe VTNZ were acting in good faith."