Vehicle engineers are combing through truck chassis designs and inspection records for flaws that might lead to failures on the road.
A Cabinet paper estimates the Transport Agency investigation could cost almost $8m but the agency's being tight-lipped about it.
It has been checking truck certifiers' files for months but would not tell RNZ the extent or seriousness of any flaws being found.
"If any specific safety risks are identified, we will contact vehicle owners immediately and issue further revocation notices," the agency's general manager of regulatory services Kane Patena said in a statement.
The chassis investigation follows on from an earlier review, begun last year, that found dodgy truck towbars and drawbeams, and disqualified three certifiers.
The chassis inquiry and recertification costs could go almost as high as the cost of that, according to the Cabinet paper, which gives an estimate of $6.7-7.8m.
The government has told the agency to make road safety its top priority, after finding its attention was too divided between safety and road building.
However, neither the Transport Minister Phil Twyford nor agency chairperson Sir Brian Roche were aware of the chassis investigation when RNZ asked them about it.
"That is not something that has come to my attention because it can be dealt with in the normal course of events," Sir Brian said, contrasting it with the closure of SH4, which was on his radar.
He had a "no surprises" policy with staff, but also trusted them to get it right.
"You can't at a point in time say, 'That's more important than anything else', you've just got to rely on people raising issues."
The chassis inquiry sprang from the towbar checks made on Auckland certifier Patrick Chu.
Reviewers found Mr Chu had been okaying trucks despite their "significantly over-stressed" chassis, then began looking at other certifiers.
At the time, truckies were warned to check chassis daily because "cracks can appear quickly", and if they found anything, to stop using the truck immediately.
Industry sources told RNZ the investigation was given added impetus by a chassis failure in the South Island some months ago. RNZ is seeking more details about that.
They said the agency had taken samples of chassis designs from most if not all certifiers, to check up on.
Chassis were usually very good at the time they were imported, sources told RNZ, but often truckies then engaged engineers to shorten or lengthen the wheel base and this could introduce flaws.
Some truck chassis would already have been fixed in order to get recertified, the agency said.
It has also been at pains to stress it is trying to get the cost of fixing chassis paid for by errant certifiers, rather than taxpayers coughing up as has been the case over truck towbars and car warrants of fitness forced recertifications.
The Cabinet paper: