New South Wales's roadside drug testing laws have been thrown into doubt after a magistrate acquitted a man who tested positive for cannabis he had smoked nine days before he was pulled over.

Key points: Man waited nine days after smoking cannabis before he got behind the wheel

Man waited nine days after smoking cannabis before he got behind the wheel THC was detected in his saliva and he was charged

THC was detected in his saliva and he was charged He had earlier been told by police to wait at least a week before driving

He had earlier been told by police to wait at least a week before driving The Greens say the judgement leaves the drug testing laws in disarray

Lismore magistrate David Heilpern yesterday found Joseph Ross Carrall not guilty of driving with an illicit drug in his blood because he mistakenly believed that he would no longer test positive to the drug.

Mr Carrall was pulled over for random drug tests in May and June last year.

When he was tested in May, he said the police officer told him he should wait at least a week after smoking cannabis before driving.

Mr Carrall said he relied on this advice when, in June, he waited nine days after smoking cannabis before he got behind the wheel.

When he was pulled over again, traces of THC were detected in his saliva and he was arrested and charged.

The arresting police officer told the court that "a line had been drawn" and that now you could be "a smoker and not drive, or a driver and not smoke" and that that was the "effect of the new laws".

Driver made 'honest' mistake thinking he could drive

But Mr Heilpern said when drug testing legislation was introduced in 2006 it was clear that "ministers had in mind that it would be drugs that were active and affect the skills that were the mischief".

"Clearly, in 2006, the technology was not nearly as advanced as it is now," Mr Heilpern said.

"Certainly it was not the aim of the ministers that if you consume cannabis [at all] you cannot drive [ever]."

Mr Heilpern said no-one was seriously contending that Mr Carrall was still affected by the drug.

He found Mr Carrall not guilty on the grounds that he had made an "honest and reasonable mistake of fact".

Mr Carrall pleaded guilty to the same offence, which he committed in May, and will sentenced on a date to be fixed.

Mr Carrall's lawyer, Steve Bolt, told the ABC that the law should be designed to improve roadside safety, not punish people for smoking cannabis.

"It's wrong, in my view, to be punishing people [by] taking their licences away when someone might have a had a smoke or two of cannabis a few days before driving a car," he said.

"It makes as much sense as taking someone's licence away for having a beer two or three days before driving a car.

"Unfortunately a lot of people would be at risk of falling foul of this legislation even though their experience of having used the drug would have had zero effect on their ability of driving a car safely."

Greens want drivers to be tested for impairment levels

NSW Greens MLC David Shoebridge said the judgement had thrown the Government's roadside drug testing campaign into disarray.

"This is significant because we've had case after case after coming before the courts, thousands of cases in NSW alone — where people have said 'I smoked a joint a day ago or a week ago and it couldn't have possibly impaired my driving and now you're going to take my licence away from me?'" Mr Shoebridge said.

"Up until now people have been thinking 48 hours or a week is safe to drive. They've been operating on that basis, thinking they are complying with the law but yet finding they can just randomly lose their licences at one of these roadside drug tests.

"This is giving ordinary people who are trying to comply with the law a chance to fight back and keep their licence."

Mr Shoebridge said legislation should be modelled on laws in the UK, where drivers are tested for their level of impairment.

He said he would be pushing for a parliamentary committee to examine the drug testing legislation.

The NSW Government said it had a zero-tolerance policy when it came to drug-driving.

A spokeswoman for NSW Roads Minister Duncan Gay said roadside tests were followed up with lab tests and 97 per cent of tests matched.

She said the research they had indicated drugs were only detected in a person's saliva for 12 hours after being ingested.