Claims that passive smoking could lead to a positive roadside saliva test are being refuted by an expert in the field.

Dr John Lewis is a retired toxicologist, who spent three decades researching drugs of abuse.

He said the technology used by New South Wales police was designed to detect the very recent use of illicit drugs.

Dr Lewis said the equipment was not sensitive enough to pick up the minute traces that passive smoking could leave.

"There are issues associated with people who are in very heavily contaminated environments, but these are quite unrealistic," he said.

"If you are standing in a room with one or two people and you passively inhale a drug, whether it's cannabis or something else, it is not going to be picked up.

"The levels are too low in that environment for it to be picked up and recorded as a positive."

There have been ongoing protests on the New South Wales north coast against saliva testing.

Protesters who gathered outside the office of Lismore MP Thomas George this week claimed the tests were producing positive results from people who had not smoked cannabis for days or even weeks.

But Dr Lewis said that was nonsense.

"Roadside tests are designed to identify very recent use of illicit drugs, and they do that reasonably well," he said.

"They're not designed, nor are they capable of detecting drugs that have been taken days or weeks ago.

"It doesn't happen."

Andrew Kavasilas, from the Australian Hemp Party, said the technology could not determine a driver's level of impairment.

Mr Kavasilas said New South Wales was lagging behind other states on the issue.

"The NT Government has just released some legislation about saliva tests and it's like a tip of the hat to us," he said.

"They've said that they're not going to take licences and they're not going to have demerit points; you just get a fine if you fail one of these drugs tests."

The state's Police Minister, Troy Grant, is currently on leave and was not available for comment.