The Immigration Department is wasting its time and taxpayers' money on forced drug tests for thousands of public servants, according to a leading workplace drug and alcohol expert.

The tests will be no deterrent, enormously expensive and might even make matters worse by forcing drug users in the department on to harder substances, according to AOD workplace testing's Dr Donna Bull.

But the department says that it is no ordinary workplace and that illicit drug use by government officials represents an unacceptable corruption risk to the nation's border protection system.

Dr Bull, an independent consultant, says up to 5000 public servants would have to be tested each year to give the department a realistic chance of catching bureaucrats who turn up to work high.

Immigration's 8500 workers were told before Christmas that they would be tested as part of an "integrity framework" for the new department, a merger of Immigration and Customs.

More than 2700 Customs officials have been tested for drugs or alcohol since testing began in 2013 and by May 2014, 10 of them had returned positive samples.

But Dr Bull says a mandatory testing regime in the British Army resulted in soldiers switching from cannabis to LSD, which is harder to detect, and similar programs in Australia's mining industry were fuelling a progression from  cannabis to methamphetamine among workers.

The Canberra-based specialist acts as a consultant to Canberra's Alcohol Tobacco and Other Drugs Association and has advised the Defence Department, the Civil Aviation Safety Authority, the Qantas pilots' union and energy sector companies on workplace drug and alcohol issues.

She told The Canberra Times that bosses who hoped drug testing would make workplace problems go away had often been disappointed.

"The evidence based for drug testing in workplaces is pretty weak in terms of its effectiveness," Dr Bull said.

"There isn't any really good evidence...that productivity will be increased, safety will be enhanced, fewer people will be using prohibited substances."

Dr Bull cited examples of mass-workplace drug testing backfiring, in Australia and overseas.

"People start using different substances, ones that metabolise more rapidly or substances that aren't on the testing panel, or they will use substances in a potentially more harmful way to get around the testing system.

"There's a lot of anecdotal evidence in the Australian mining sector, which does quite a lot of drug testing, that miners have switched their drug of choice from cannabis to methamphetamine."

Dr Bull said a typical testing frequency, of 10 per cent of employees each year, was "no deterrent at all" and unlikely to change behaviour.

"If you want people to change their behaviour, there needs to be a real or perceived likelihood of being apprehended - around a one in three chance ," she said.

"They'll need to be testing about 5000 people a year, the expense is enormous and the cost-effectiveness is not good."

But a departmental spokeswoman said the testing program made sense both for safety and integrity reasons.

"The Drug and Alcohol Management Programme (DAMP) serves both health and safety and integrity goals," the spokeswoman said.

"The ACBPS recognises that organised criminal groups seek to infiltrate law enforcement agencies and corrupt public officials, and the DAMP is one of a suite of integrity measures designed to mitigate this risk."