Towbar makers are warning if NZTA fails to front up with money to cover the latest slew of failures, it will set a precedent for passing the costs on to motorists.
A trio of the Auckland manufacturers have joined together, and with the backing of dozens of small businesses, are intent on forcing the agency to pay.
The agency has told 1400 vehicle owners they will have to pay themselves to get their towbars recertified. It revoked all the towbars certified by Auckland engineer Patrick Chu.
The affected vehicles included trucks, motorhomes and vans, and the agency said there was a risk that trailers could detach from the vehicles.
Aro Brothers, Counties Commercial and Acker Bilt have between them made 1000 towbars. All lost their safety certifications just before Christmas under the Transport Agency safety alert, and many remain uncertified and illegal to use.
Customers were hurting, but the bureaucrats did not care, a co-director of Acker Bilt in Penrose Jeremy Moore said, evidenced by the agency writing to his firm just once.
"They've obviously just thought, 'well, we don't want to pay for it and hopefully if we just let this one just slip by, nobody will do anything and people will pay for it themselves'.
"But we know if we allow it to happen there's more to come basically," Mr Moore said.
He accused officials of being blase, pointing out the safety alert was issued in December, more than two months after certifying engineer Chu was suspended.
Even since the alert, there had been a time lag.
"The one email correspondence that we had actually stated that all the customers had finished being spoken to," Mr Moore said.
"That's simply incorrect - most people are only finding out in the last week or two."
The trio of companies is waiting for the agency's response to its open letter demanding that it pay instead of them or their hundreds of customers. Recertifications cost $400 or so and upwards.
The agency has previously paid up to re-certify thousands of truck towing connections mostly in the South Island, and for thousands of warrants of fitness issued by garages since suspended for breaking safety rules.
The 1400 towbars certified by Mr Chu are not covered.
Rebecca Hadley of Dingo Groundworx in Auckland has three trucks she cannot use to tow diggers - half her fleet - and costs are mounting.
"Oh God, to be honest I don't think I'd want to do the sums because it'd be enough to make me cry," she said.
"Not to mention it's all the cost as well of trying to ring around and finding a certifier to recertify them, and they seem to be few and far between."
Worse still, Ms Hadley had got a confusing message from the Transport Agency that the tie-downs on her trucks needed recertifying too, but had not been able to confirm this.
Blair Johnson has 15 motorhomes sold by his company Jayco Auckland caught up in the revocations, and unhappy customers on the phone.
"It's caused them a great deal of distress. When they were notified of the problem many of them were on holiday, they were towing boats, towing smaller vehicles and were really concerned about how they would actually get home, knowing that their vehicle was now illegal," Mr Johnson said.
"It's quite concerning that their [Transport Agency] key focus was on safety, yet some people were notified before Christmas, some are still being notified now. You do have to wonder, if it was a serious issue, why they didn't act quicker."
Mr Johnson believes the Transport Agency's failure to pay to cover its own auditing failure amounts to "preying on" retired people just because they do not use their motorhome for business.
Jeremy Moore said the three towbar makers' next step, if the agency will not pay, is to pursue a claim against the insurer of the certifying engineer covering all 1000 towbars as a group.
Individual claims would founder on the $5000 excess.
The Transport Agency said it was considering the letter from the towbar makers.
"Safety is the ... top priority which is why the decisions to revoke some certifications have been made. We know that this will unfortunately impact on some businesses and individuals," it said in a statement.
Figures newly released to RNZ show the agency cut its compliance workforce from 54 in 2010 to just 28 last year, including laying off five such staff in 2015.
The agency is now hiring a whole lot back to enforce the rules.