The number of workers found with opiates in their system has soared in the Bay of Plenty.

In figures released from The Drug Detection Agency, random workplace testing in the Bay of Plenty from January 1 to June 30 this year found 557 tests were positive for drugs.

Agency chief executive Kirk Hardy said the random testing was based on safety sensitive areas of the workforce while pre-employment testing encompassed all industries including professionals, retail and blue collar.

Of this, 11.6 per cent of people tested positive for opiates, an increase of 26 per cent from last year.

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"Bay of Plenty sits at a 5.4 per cent positive rate for all random tests conducted in 2016, and year to date this figure is 4.84 per cent," Mr Hardy said.

"It is a good sign, however . . . [as we] become busier leading up to Christmas and the holiday season, unfortunately, the trends in the past see the positive rates also rise."

Mount Maunganui GP Tony Farrell, who specialises in addiction support, there was already a rise in prescription opiate taking "which is a concern in New Zealand".

However, some of the opiate figures would include innocent examples of painkillers like Panadeine or medication prescribed by a doctor which could skew results, he said.

"It's not all clear cut. The majority of them will be illicit use but it's worth knowing there are some medications that can interfere and give false positives."

Dr Farrell said he was surprised the drug tests did not appear to act as a deterrent to some drug users. He expressed concern that drug testing could take a person's job away from them when they actually needed support and he credited the increasing number of employers he was aware of who had drug policies that worked to get an individual help and support instead of immediate dismissal.

The number of non-negative pre-employment tests in the Bay of Plenty grew from 5.4 per cent in 2016 to 7.09 per cent this year to date - translating to 31.2 per cent.

New Zealand Council of Trade Unions president Richard Wagstaff said drug testing was appropriate when people worked in transport or heavy machinery but he felt money could be better spent on other health and safety issues "with greater effect and less impact on worker's privacy".

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"We are worried that over-zealous drug testing may result in people switching from cannabis (which stays in the system longer) to less detectable but more serious drugs like methamphetamine."

Mr Wagstaff said drug use in the workplace was an issue but the council felt it received disproportionate attention compared to other sources of harm such long working hours.

"Many of the industries which religiously drug test workers (such as agriculture, horticulture and forestry) also work some of the longest hours in New Zealand. We call for industries to look carefully at all causes of impairment."

Mr Wagstaff encouraged employers thinking about introducing drug testing policies to seek independent legal advice on their implementation.

The number of workplace serious harm notifications in the Bay of Plenty has gradually dropped in recent years from 458 in 2011 to 213 in 2015 and 95 for the first four months of 2016, in the latest figures from Worksafe New Zealand.

A WorkSafe spokesman said drug testing was one part of how a company might address impairment in their workplace but declined to comment further.