Newly-released information shows checks on engineers who inspect big trucks plunged at the same time as the number of large rigs on the road jumped.
The NZ Transport Agency (NZTA) says there is no evidence to suggest a link between fewer certification checks and crash rates.
But the agency said it remained concerned the road toll was creeping up and that the proportion of the toll of death and injury involving trucks remained high.
"While the long-term trend in New Zealand had seen road fatalities drop by over a third from 2000 to 2014, the increase in road trauma since 2015, including crashes involving heavy vehicles, is very concerning," NZTA said in a statement.
Another factor is the industry's shortage of drivers. Recently warnings have been reported, including from a driver, that there could be more crashes due to more inexperienced drivers driving bigger rigs.
The agency cut its heavy vehicle compliance staff numbers in half in 2014. Figures released under the Official Information Act show that led to the number of audits it was doing on truck-certifying engineers - who both check and design things such as towing connections and brakes - plunging from 70 a year before 2014, to just 30 a year since then.
The discovery of a mass of poor certifications, and a spate of cracked or weak towing connections, has massively disrupted the industry this year.
At the same time, the government's strategy for upping freight efficiency has led to the number of big 50-tonne trucks on the road jumping by up to 30 percent a year.
"These longer or heavier combination vehicles are the next generation of trucks, designed to carry more goods with each trip," the agency said in its 2013-14 annual report.
"They have more advanced safety features than the older trucks they are replacing. With fewer truck trips on our roads, the crash risk is also lowered."
But the proportion of the road toll contributed by truck crashes had climbed, from about 15 percent between 1983 and 1999, to about 20 percent now, though actual death numbers are down, from around 1800 total to 1200 for those periods.
"There are a range of potential factors likely to have contributed to the recent increase in serious crashes involving all types of vehicles, including a significant increase in overall vehicle kilometres travelled on New Zealand roads," the agency told RNZ.
However while there was a big drop-off in the number of fatal crashes per million kilometres trucked from 2000 till 2008, since then this had levelled off.
"There is no evidence at all to suggest any link between recent changes in heavy vehicle crash rates and the number of certifier audits," the agency said.
It is, however, now recruiting three more engineers to do the audits that previously just one engineer had been doing nationwide.
By last year, almost half of all certifiers were failing the few audits that were carried out.
Thirteen engineers out of the 28 audited scored low enough to trigger a follow-up random check; and eight of those scored so low they faced a full re-test.
These fail rates were much lower in 2015 and 2016.
Dick Joyce, one of two certifiers recently suspended, scored just 0.7 out of a possible 3 marks in his latest audit in March this year (anything under 2.4 triggers re-testing). He wasn't suspended until June.
The new figures also show the problems go back years: Since 2009 fully two-thirds of the engineers told by the agency to shape up, failed to do so by whatever deadline they were given.
Yet, few heavy vehicle certifiers have ever been suspended; just seven in all that time, in an industry numbering approximately 170.
As for the number of crashes linked to inadequate truck towing connections, the OIA information revealed little.
The police told RNZ they didn't keep such records.
The Transport Agency said it was able to draw on police crash reports to conclude there had been only one death due to an inadequate truck tow connection in the last two decades, and fewer than 20 injuries. It had recorded 86 non-injury crashes of this type since the agency was set up in 2008-9.
However, industry sources say these figures reflect a lack of record-keeping about the causes of crashes, beyond speed or alcohol, more than anything else.
The trends for truck crashes as a whole are the opposite of what the agency has had as its stated aim for years.
It remains unclear from parliamentary records about the Transport Agency's restructures, when and how it alerted MPs to its aim of cutting back its heavy vehicle compliance unit in 2014.