The union, which represents heavy vehicle safety inspectors, has warned the NSW roads minister, Melinda Pavey, that the state government's decision to "not fill a significant number of ... positions" is a critical road safety issue.

There has been a spate of fatal truck crashes in NSW, including three deadly collisions in two days earlier this month, and preliminary figures reveal there was a 45 per cent increase in truck-related deaths in the past year.

In a letter sent to Ms Pavey on Friday, the union's assistant general secretary Troy Wright said the union's figures showed there were 34 fewer safety inspectors than at the same time last year, a 15 per cent reduction of the total workforce.

"The explosion in heavy vehicle accidents in NSW tragically corresponds with the NSW Government's decision not to fill a significant number of heavy vehicle inspector positions," the letter said.

"The result is that truck companies are now very much aware that if a vehicle is not up to scratch it will probably go undetected until there is a major accident, which have been all too horrifically common of late."

The minister's office referred questions to the Roads and Maritime Services, which acknowledged a decrease in staff numbers, but not the number identified by the union.

"At the beginning of last year Roads and Maritime Services employed 241 enforcement operations inspectors," an RMS spokesman said.

"There are currently 231 enforcement operations inspectors following retirements and resignations which have occurred in the past 12 months, a less than 5 per cent reduction."

The spokesman said NSW maintained "automatic monitoring and screening systems" across key high traffic roads that target the highest risk areas and vehicles.

But Mr Wright disputed the government's figures, and said many positions had remained vacant for at least a year.

"The government's decision not to fill heavy vehicle safety inspectors positions from our highways is shockingly irresponsible," Mr Wright said.

"We warned the state government the loss of these roles would put lives at risk on our roads."

Mr Wright said the state's heavy vehicle safety stations were not always staffed because of the shortages.

"We stand by our figures and we believe the vacancies make up about 15 per cent of the workforce," Mr Wright said.

Data from the NSW Centre for Road Safety shows the number of fatalities related to heavy truck crashes climbed from 56 in 2016 to 81 last year.