Fewer workers are failing workplace drug tests in the central region, which includes Whanganui.
Drug Detection Agency figures show that only 4.19 per cent of all workplace drug tests were positive in 2015 compared to 4.85 per cent the previous year.
Hayden Loader, general manager of Whanganui construction company Loaders, said he believes workplace drug testing is working.
Loaders has a mandatory pre-employment drug test as well as random and post-incident testing, and he said the company's clients could also require their own testing.
Mr Loader said tests kept workers clean and showed the company was serious about drug issues.
Workers who tested positive for drugs would leave work until they could provide a clean test, then be given a final chance, he said.
A group of workers on their final chance had recently been tested and everyone had passed.
"I think it works on all sorts of levels."
Loaders' staff worked with heavy machinery and staff needed to know the person operating the machine wasn't under the influence.
"We just can't afford to take the risk," said Mr Loader, whose family firm has more than 60 years' civil construction experience in the Whanganui district. He said the stigma around reporting colleagues who might be under the influence was slowly disappearing and the company always acted on workers' concerns.
Drug testing was becoming more common in workplaces, he said, with mandatory drug tests becoming the norm.
Drug Detection Agency chief executive Kirk Hardy said there had been a decrease in the overall percentage of positive drug tests in forestry reflecting a behavioural shift to an intolerance for drugs in such industries.
He said the industry was vigilant with testing and education programmes.
As well as preventing workplace accidents, the industry was addressing issues before they became a wider problem in society.
Mr Hardy said the rate of positive drug tests was also down in the transport industry.
It was normal practice for transport operators to have stringent drug and alcohol testing and to have the agency regularly present to members about the dangers of drugs in the workplace.
However, the proportion of people testing positive for methamphetamine was up in both the forestry and transport industries.
Mr Hardy said that was a reflection of a national trend of methamphetamine becoming a much larger issue in New Zealand in general.
The agency conducted more than 120,000 drug tests nationally in 2015.