The proportion of positive tests showing synthetic cannabis use dropped from 1.9 per cent to 1 per cent in Bay of Plenty.
Ballance Agrinutrients spokeswoman Edith Sykes said the Tauranga-based company had a drug and alcohol policy to ensure that it was a safe workplace.
Ms Sykes said the increase in workplace drug use throughout the Bay of Plenty did not reflect the company's experience but she said it was important for workers in the industry to stay drug-free.
"From our people making fertiliser and animal feed, to our sales team out on the road, to our office-based teams - we want to send everyone home safely each day. We also have an obligation to manage all workplace hazards and risks, and to make sure all of our people are protected from the potential actions of someone affected by drugs or alcohol on the job."
In 2014, the company dismissed two employees who were under the influence of marijuana during working hours, she said.
Nationwide, positive workplace drug tests increased slightly last year on the previous one from 6.14 per cent to 6.19 per cent.
Methamphetamine showed up in 11.8 per cent of positive tests, up on 8.1 per cent in 2014.
TDDA chief executive Kirk Hardy said the methamphetamine problem was not new to New Zealand and there appeared to be a growing supply of - and demand for - methamphetamine.
The trend was similar to the late 1990s and early 2000s.
He believed the social impact would be a resurgence of "horrific unspeakable" crimes.
Mr Hardy said there had been a decrease in the overall percentage of positive drug tests in forestry - reflecting a behavioral shift to an intolerance for drugs in such industries.
He said the industry was vigilant with testing and education programmes.
As well as preventing accidents in the workplace, 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.
He said it was normal practice for transport operators to have stringent drug and alcohol testing and to have TDDA regularly present to members about the dangers of drugs in the workplace.
However, the proportion of people testing positive for methamphetamine was up in 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.
TDDA conducted more than 120,000 drug tests nationally in 2015.
