A heavy vehicle engineer kept quiet about deficiencies in his designs for years until a trailer snapped off a truck, according to NZTA documents.
The Transport Agency put out a safety alert about the engineer, Dick Joyce of Wellington, earlier this month, adding to widespread disruption in the transport industry. It calls for urgent checks on the towing connections of almost 500 heavy vehicles including big trucks and logging trailers.
But Mr Joyce said it was a "witch hunt" and he had never knowingly endangered the public.
He expressed disbelief at what he said was a summary of the current inquiry that the Transport Agency had just given him, that faulted 80 percent of his 480 or so truck towing certifications over the last decade.
It was "stupid", he said.
Transport Agency documents now released under the Official Information Act chart an investigation into Mr Joyce a decade ago, after a trailer broke off a truck and rolled backwards near Taupō.
Investigations from 2007 to 2009 found the stresses on the steel were three times the maximum allowed.
The documents say Mr Joyce knew about the deficient design as early as 2005, when he came across cracks in another truck's drawbeam, but that he did not raise a red flag about it to anyone else, even though at least nine other large and medium-sized trucks had the same flawed design.
"Joyce failed to take reasonable steps to safeguard the health and safety of people ... These actions present a significant risk to land transport safety," the 2009 OIA documents said.
Dick Joyce rejected not only that finding, which he said he had disputed at length at the time, but also the current investigation into his work that has triggered alarm bells.
"I've never taken a shortcut," he said.
"I've had probably 15 calls from people saying they don't believe a word of it [the current inquiry], they agree that there's a witch-hunt.
"I believe Land Transport [Transport Agency] are being totally unfair and they're totally biased."
The OIA documents said Mr Joyce modified the design but broke certification rules by not alerting the agency, or telling truck owners or Certificate of Fitness inspectors about the "need for extra vigilance".
"I was aware of the deficiency but I decided the deficiency was not catastrophic," Mr Joyce told RNZ.
"It would fail gradually and everybody would see that it had failed - it would crack slowly."
The 2007 trailer failure was not due to the flawed design or cracking but because the truck driver probably missed a gear, and rolled backwards into a ditch, snapping the drawbeam, he said.
But the police's crash investigation unit agreed with the Transport Agency, that it snapped while being towed.
This mirrors a similar trailer failure a year ago near Murchison, that sparked the extra scrutiny of certifiers that has since tripped up Mr Joyce and forced off the road 1500 heavy vehicles certified by Nelson engineer Peter Wastney.
The Institute of Engineers - IPENZ - also found against Mr Joyce.
The OIA documents said an engineer who certified at least 10 trucks with the flawed design reported that Dick Joyce never told him about the cracking.
"Not advising anyone ... can only be considered as negligence and most likely conduct unbecoming ... Joyce's actions fell short of the duty of care required of a professional engineer."
Mr Joyce, however, said he thought there was only one flawed drawbeam - but that a contractor had kept using his old, deficient drawings to make others, without him knowing.
Mr Joyce scored zero out of three for drawbeam calculations in a 2008 agency audit. Engineers must score above 2.4 to avoid extra auditing.
The 2009 investigation resulted in his suspension for three months. He did not appeal. "I had already spent a huge amount of time on this and decided that I needed to put it behind me and get on with life."
He regained his certification quickly, he said, and had passed six or seven audits since, all above 2.4.
Now, though, his business in Seaview, Lower Hutt, was suffering and his reputation was on the line, he said; he had just got a summary from the Transport Agency of its investigation, and it was harsh.
"I'm prepared to accept that I'm not perfect and every now and then I'll make a mistake.
"However, this is the stupid part about it, they gave me a summary and ... if I take it as literal, 80 percent of my work was at fault" in certifying 480-odd towing connections in the last 10 years.
The agency is also investigating whether some heavy vehicle certifiers have been signing off on engineering work based just on photos, without checking it firsthand.
Dick Joyce said he had never done that, but had seen "indications" it had occurred. He would not comment further about that.