A new zero-tolerance policy towards drugs and alcohol is being opposed by staff in a South Australian Government department amid concerns about plans for workplace testing and whether there's any legal authority behind the move.

For months, the Public Service Association has been pushing back against a policy that all employees of the Department of Planning, Transport and Infrastructure (DPTI) must not have any amount of alcohol or illicit drugs in their systems while at work.

The PSA said enforcement of the policy would involve some type of testing regime, the precise details of which it said remained unclear.

PSA acting general secretary Natasha Brown said while DPTI had backed down on urine testing being part of the workplace tests, members remained opposed to a number of aspects of a policy package she described as "well short of ... reasonable".

"The focus of drug and alcohol testing should be about the capacity to perform duties consistent with the requirements of the job — and not about personal choices, lifestyle or irrelevant medical treatment," she said.

"People need to be supported in achieving and promoting safe and healthy working environments — and should not be subjected to an intrusive or punitive regime.

"The PSA intends to hold a joint union members' meeting which may result in further action."

The department's arguments for the policy have included a desire for a "safety-first culture" in "a modern public service" and wanting a "consistent approach to all staff across our department".

Train drivers are already subject to testing. ( ABC: Ashley Walsh )

But the PSA has questioned whether the department has the legal authority to impose testing on all staff.

Rail safety legislation requires rail safety workers such as train drivers to submit to testing.

But the association said legislation governing DPTI's non-rail safety staff did not give the department authority to test those employees.

"PSA members are covered under the public sector act which has no provision for random drug testing," PSA secretary Nev Kitchen said.

"They're not involved in the use of machinery or driving vehicles."

Mr Kitchen said a DPTI manager already had the ability to send an employee home if they believed they were actually affected by drugs or alcohol — rather than merely having the substances in their system.

DPTI CEO says zero-tolerance 'reasonable'

In a statement sent to staff earlier this year, DPTI CEO and Railways Commissioner Michael Deegan has said there had been "a long period of consultation" and the department would be making "some changes based on that feedback".

"However, the intent of the policy remains the same," he said.

"We believe it is reasonable and right to have a zero-tolerance policy against illicit drugs and alcohol, with an accompanying education and awareness program to support its introduction.

"We are confident this measure will save employees from injury and harm due to the application of poor judgement brought about by the consumption of drugs and alcohol. It is also plays a role in the department's overall commitment to employees' health and wellbeing.

"The policy is not designed to be punitive."