Documents show the Transport Agency (NZTA) was calling problems with truck-trailers a significant safety risk for months before it pulled the trailers off the road.
They show the alarm bells were ringing loudly at NZTA after a poorly-designed trailer snapped off on a highway.
The agency only released the hundreds of pages of emails and reports after RNZ went to the Ombudsman.
The documents tracked the investigation and suspension of Nelson heavy vehicle certifier Peter Wastney, who eventually quit.
He had designed a trailer drawbeam that snapped off a truck near Murchison in August 2017, then hit a bank.
The agency knew he had a poor record as a certifier going back 17 years; an audit just before the 2017 crash faulted him over things like not accurately recording what welding was done.
He was suspended a fortnight after the crash.
But by then an internal NZTA report was already warning that unsafe trailer certifications "may be being made on a daily basis" by Mr Wastney.
Between then and December 2017, the Official Information Act response shows NZTA managers repeatedly said they believed there was a significant risk to the public, because 1500 truck-trailers designed or certified by Mr Wastney were still on the road, mostly at the top of the South Island.
Emails show Mr Wastney told investigators the Murchison trailer was a one-off design, but they then discovered a second similar trailer, which had also failed earlier while in service.
At this point, in December 2017, an agency manager said Mr Wastney's "ethics and integrity" and "competence" were in serious question and he was "a significant risk to road safety".
Even so, with the Christmas holidays and busy roads approaching, the agency opted to wait.
But in February 2018 that hiatus came to an end, after a truckie near Nelson spotted a crack in his towbar.
"The crack is significant and rapid failure was imminent," an engineer told NZTA, adding the design was "very unusual".
"The driver deserves a bonus for doing a correct safety walk around at the start of the day. The drawbar is to be scrapped. Under no circumstances is it to be repaired."
On 9 February an NZTA manager warned that this towbar crack - plus the agency's "existing concerns about most (if not all)" of Mr Wastney's other files - "means there could be some extremely unsafe vehicles in operation right now" with the potential to cause death or serious injuries.
But the agency already knew this: In early December 2017, a manager had noted that six out of 16 Wastney trailers looked at could have been predicted to fail; and that some were built with 6mm instead of the specified 10mm steel plate.
Also, immediately after the Murchison crash, engineers had labelled that design "seriously deficient" and "an immediate risk to the public".
The public only learned some of this in February 2018, after the Nelson towbar discovery, and only because RNZ got wind of an NZTA safety alert issued to the trucking industry.
The alert itself belied the months of loud alarms: It just told truckies to clean and check any Wastney towing connections. It did not take them off the road or order recertification by engineers.
Six weeks later, the OIA shows the agency was worried because only 26 of the 1500 vehicles had been recertified, and it was asking itself if it had responded adequately.
A month later, it revoked the certifications of more than 800 Wastney trailers.
The managers had discussed how such a revocation would cause massive industry disruption at the top of the South Island.
Several other truck certifiers have subsequently also been suspended and their work revoked
The Wastney case snowballed during 2018 into scrutiny of other regulatory areas, such as car warrants of fitness, and to suspensions and revocations covering about 50,000 vehicles, and scores of transport licence holders, garages and certifiers.
Certifiers have told RNZ that since then, the oversight of the heavy vehicle certification had improved a lot, though there was significant stress from all the changes coming in.
In its latest move, NZTA has just told certifiers it is about to begin physical inspections of trucks, to make sure they match what's written in their safety paperwork by the certifiers, after months of doing spot checks on those files.
The agency's new regulatory manager Kane Patena in a statement said the agency acknowledged its approach had not been strong enough.
"The NZTA previously operated a high-trust, devolved regulatory regime, focusing on ongoing education ... That approach was not strong enough to ensure high levels of regulatory compliance.
"We are implementing a best practice regulatory model which prioritises safety."
Compliance had been toughened up over the past nine months and capability had increased, Mr Patena said.
- 2000 - 2017 - Series of generally low-scoring NZTA audits of certifier Peter Wastney
- June 2017 - Audit with unsatisfactory outcome, including welding not accurately recorded and drawing and analysis missing
- 27 August - Truck-trailer drawbeam breaks near Murchison and trailer crashes
- 7 September - NZTA investigators say they are "concerned that further unsafe certification determinations may be being made on a daily basis"
- 18 September - Mr Wastney suspended for three months
- Before December - NZTA discovers a second similar Wastney drawbeam that failed in service
- 8 December - Managers say in all files reviewed "there is insufficient evidence ... to justify a competent certification outcome"
- December to January - Holiday break
- Early February 2018 - Towbar crack discovered near Nelson
- 9 February - NZTA manager writes: "The fact that rapid failure was imminent ... plus our existing concerns about most (if not all) of his other files, means there could be some extremely unsafe vehicles in operation right now with the potential to cause DSI to the driver and/or other road users."
- Late February - NZTA issues safety alert, with advice to get towing connections cleaned and inspected
- 29 March - Agency records "concern that 6 weeks after issuing the safety alert, only 26 of the 1502 vehicles have been .... recertified"
- 23 April - Agency revokes more than 800 Wastney towing connections