A report written by Labour Department senior medical practitioner Chris Walls has questioned the usefulness of testing workers for drugs such as cannabis.

Workplace drug testing is common in the forestry, fishing and transport industries.

Occupational Safety and Health is asking for feedback on the report before putting together a policy.

In his paper, Dr Walls said there were "no useful analytical measures to accurately indicate the level of acute impairment" caused by cannabis.

Urine testing "has no practical use in assessing impairment caused by marijuana use," he says.

Urine tests showed whether someone had used drugs, but it could have been weeks ago. He said blood serum tests, analysed by gas chromatography, could help show those who had smoked cannabis in the previous 12 hours. Dr Walls also recommended that drug and alcohol testing be handled by medical professionals, not human relations departments.

In October last year, 20 police officers raided a Carter Holt Harvey mill in Tokoroa and, with the permission of management, had workers sniffed by a drug dog. No drugs were found. Health and safety officer for the Engineering, Printing and Manufacturing Union (EPMU) Mike Ward said New Zealand employers used drug testing in a punitive way.

Cheap urine testing was "a trendy way [for employers] to be seen to be positive about health and safety," he said. "They use valuable resources purely to get an answer to the question 'Have they smoked a joint in the past week?"'

Employers Federation chief executive Anne Knowles said employers had worked with Dr Walls in putting together drug testing guides, and knew about the link between testing and impairment. She said employers used testing only in a very limited and non-punitive way.

EPMU general secretary Andrew Little said even if a collective employment contract allowed drug testing, case law supported an employee's right to refuse. Workers with individual contracts could also mount a good argument against it.

Fletcher Challenge Forests spokeswoman Jacqui Miller said the company tested urine for opiates, cocaine, amphetamines and cannabis before hiring a worker, after an accident and if they seemed impaired. Samples were tested by the Institute of Environmental Science and Research, which gauged impairment. If drugs were present, but not enough to impair, ESR did not tell Fletchers.

Air New Zealand technical and flying crew are tested for drugs during routine medical check-ups.

The chief executive of wood products at Carter Holt Harvey, Devon McLean, said drug testing was done only before hiring a new worker and after any accidents. None of these three companies used random tests.