A magistrate has launched a blistering attack on the New South Wales Government's roadside drug testing regime, challenging the accuracy of its claims that cannabis can only be detected in a person's saliva for up to 12 hours.

Key points: Drivers are being charged days after using cannabis, magistrate says

Drivers are being charged days after using cannabis, magistrate says Magistrate points to hundreds of cases of this happening

Magistrate points to hundreds of cases of this happening He said no evidence had been presented in court showing drugs can only be detected for up to 12 hours

In a judgement on Monday, Lismore magistrate David Heilpern said he had heard hundreds of cases over the past few months in which drivers said they had waited days, sometimes weeks, after smoking cannabis before driving.

Yet they had still tested positive to cannabis and been charged with the offence of driving with an illicit drug in their blood.

"In the vast majority of cases the time frame has been over 12 hours," Mr Heilpern said.

"On not one occasion has the prosecution cavilled with this contention."

Mr Heilpern said the prosecution had also remained silent when people claimed they had tested positive to cannabis after consuming the drug by passive smoking, eating hemp seeds, rubbing hemp balm or taking medicinal tincture.

"The prosecution have remained silent when people claim that they consumed cannabis weeks prior," he said.

"Not once has any scientific evidence been produced to this court that supports the contention that the final or any other test only works for 12 hours.

"It could be that every single one of those defendants are lying to the police. However, on balance, I find that this is unlikely."

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Laws intended to stop drug-affected drivers

Mr Heilpern said when the Road Transport Legislation Bill was introduced in 2006, "Parliament did not intend to stop people from driving or take away their licences three days or six days after they had consumed cannabis".

"It is clear ... ministers had in mind that it would be drugs that were 'active' and 'affect the skills' that were the mischief."

Official advice on drug use: Illegal drugs can be detected in your saliva for a significant time after drug use, even if you feel you are OK to drive

Illegal drugs can be detected in your saliva for a significant time after drug use, even if you feel you are OK to drive The length of time that illegal drugs can be detected depends on the amount taken and frequency of use of the drug

The length of time that illegal drugs can be detected depends on the amount taken and frequency of use of the drug Cannabis can typically be detected in saliva by a drug testing stick for up to 12 hours after use

Cannabis can typically be detected in saliva by a drug testing stick for up to 12 hours after use Stimulants (speed, ice and pills) can typically be detected for one to two days Source: NSW Centre for Road Safety

Mr Heilpern made the comments while sentencing two men who had pleaded guilty to two counts each of driving with an illicit drug in their blood.

Riley Vincent Garlick-Kelly, 22, was pulled over twice for random drug testing late last year.

He told the court he smoked cannabis three to four days before testing positive in October 2015 and six days before testing positive in November 2015.

Mr Heilpern found that the offences "were not within the mischief sought to be cured by Parliament" and placed Mr Garlick-Kelly on a two-year good behaviour bond.

Darrell James Squires, a 38-year-old single father, told the court he smoked cannabis every night because he suffered from insomnia and anxiety.

When he was pulled over in May 2015, he returned a negative test result and was allowed to drive away.

He was pulled over again in July 2015 and this time tested positive.

Mr Heilpern accepted that Mr Squires believed his practice of smoking cannabis before bed had led to a negative test.

Both men were given two-year good behaviour bonds, with no convictions recorded.

Man who tested positive nine days after smoking acquitted

It is the second time Mr Heilpern has criticised the mobile drug testing program.

Last month he acquitted a man who tested positive for cannabis he had smoked nine days before he was pulled over.

Joseph Ross Carrall was found not guilty of driving with an illicit drug in his blood because he mistakenly believed he would no longer test positive for the drug.

In his judgement this week, Mr Heilpern noted that since media coverage of his decision in Mr Carroll's case, Transport NSW had added the word "typically" to its advice that "cannabis can be detected in saliva for up to 12 hours after use".

NSW Greens MP David Shoebridge said the judgement was further proof of the Government's "evidence-free" approach to drug policing.

"If they have any evidence to support their claim that cannabis leaves the system after 12 hours, it needs to be presented to the court. They need to put up or shut up," he said.

But the Centre for Road Safety stood by its current advice.

Its executive director Bernard Carlon told the ABC cannabis could be detected at the roadside in oral fluid for up to 12 hours after use "depending on the quantity and the potency consumed".

In a statement, NSW Roads Minister Duncan Gay told the ABC: "It is illegal to drive with the presence of illegal drugs in your system — it is that simple — if you are caught drink or drug-driving the law is clear".

He said in 2014, 16 per cent of fatalities involved drink-driving — the same proportion as those that involved drivers with illegal drugs in their system — which is why the Government was tripling drug testing.

"My advice is don't take illegal drugs and if you do, be responsible and conservative with your decision of when it is safe to drive to avoid the consequences of drug-driving charges or worse, harming innocent road users because you are impaired," Minister Gay said.

"Much like there is research and guidelines around how long it takes alcohol to leave your system, if you choose to take drugs, guidelines and advice is available [developed by medical experts and drug detecting manufacturers] for when drugs should be out of your system."