In a decision handed down in Lismore Local Court on Monday, Magistrate David Heilpern​ said the lack of information around testing levels was part of the "mystery and uncertain-by-design of the current testing regime."

Magistrate Heilpern made the comments as he dismissed a drug-driving charge against Joseph Carrall, who tested positive for cannabis in June last year, nine days after smoking the drug.

The court heard Mr Carrall had relied on advice given to him by a police officer during another roadside drug test, one month earlier, where he also tested positive to cannabis. He has pleaded guilty to this charge.

Magistrate Heilpern found that on that occasion in May 2015, Mr Carrall was told by Senior Constable Chayne Foster he must wait one week after smoking the drug in order to be "fine to drive."

When he was arrested for a second time by Senior Constable Foster on June 23, 2015, after testing positive for cannabis during a another random roadside test in Lismore, he said "I thought I would be alright, it was over a week ago," the court heard.

While Magistrate Heilpern noted the law required only "the mere presence of a minute or residual presence of THC" for the offence to be proved, he found Mr Carrall not guilty of the June offence because, based on the officer's advice, he had "honestly believed that the cannabis was no longer present."

He found Mr Carrall was "telling the truth when he says that the last cannabis he smoked was at least nine days prior and he believed all the cannabis would have been gone from his system."

On those grounds, the defence of "reasonable and honest mistake of fact" should be applied, he said.

As part of his decision, Magistrate Heilpern said the lack of available information around the tests made it reasonable for Mr Carrall to have relied upon the officer's advice.

"After all, how else is a person to determine when they are right to drive?," he said.

The finding that Mr Carrall tested positive to cannabis nine days after smoking it is at odds with the official advice provided by the NSW Centre for Road Safety.

"Based on medical research and the manufacturers' specifications of the device we use for mobile drug testing, cannabis can be detected at the roadside in oral fluid for up to 12 hours after use, depending on the quantity and potency consumed," the centre's executive director Bernard Carlon said in a statement.

Whereas roadside breath testing for alcohol is based on concentration thresholds – under 0.05 is the legal limit for most drivers in NSW – drug-driving tests only screen for the presence of the cannabis, speed or ecstasy in any concentration

Mr Carlon said all positive roadside drug tests were subject to further laboratory testing before charges were laid.

"Around 97 per cent of samples collected from drivers who have tested positive to a roadside testing are confirmed positive in the laboratory."

Acting Assistant Commissioner Stuart Smith from the Traffic and Highway Patrol said the NSW police were "reviewing the court's decision."

While local court decisions have no precedent value, the case was significant in revealing the lack of clarity around how the tests work, Mr Carrall's lawyer Steve Bolt said.

"The police will not release the calibration of their testing device. No one knows what police are testing for….the police themselves have no idea how it works.

"It's not a floodgates situation. It doesn't mean no one is ever going to be found guilty again...but I would think that other magistrates would be willing to adopt the same approach."

A finding of not guilty for drug-driving offences is a rare occurrence in NSW courts, with just four of the 3043 drug driving matters heard between January and September last year resulting in not guilty findings.

The number of drug driving cases heard in NSW courts increased 109 per cent over the same period compared with 2014, according to data from the NSW Bureau of Crime Research.

The court's decision comes as the NSW police crack down on drug-driving, following the announcement by Deputy Premier Troy Grant in December that roadside drug testing would triple to 97,000 tests each year by 2017.