A saliva swab that tests for drugs in drivers.

A saliva swab that tests for drugs in drivers. Photo: Ken Irwin

The ACT's new drug-driving laws have come under fire after a senior public servant who ate a breakfast muesli containing hemp seeds was dragged through a nine-month court battle.

The 2011 laws raised immediate concerns from lawyers and civil libertarians for their zero-tolerance approach, which criminalises even the smallest trace of drugs found in a driver's system.

Fears were raised that the laws, designed for road safety, ignored the actual impairment level of a driver and threatened to snare Canberrans who had accidentally or innocently ingested drugs.

That approach differs to drink- driving laws, where drivers are allowed to consume a legal minimum because it does not impair their ability to drive.

On Friday, drug-driving proceedings against a 37-year-old Canberra woman were dismissed in the ACT Magistrates Court.

The case, which Magistrate Beth Campbell said had ''fascinated'' her, arose after the woman was involved in a collision while riding her motorbike in Barton in January.

She suffered significant injuries, and was taken to Canberra Hospital for treatment. A mandatory blood test returned a positive result for a very low level of THC, the chemical that gives marijuana its psychological effects.

Police showed up at the defendant's doorstop and, much to her surprise, informed her marijuana had been detected in her blood stream. The woman, who has driven for 20 years without a blemish on her record, told police that was impossible, because she hadn't smoked cannabis for 15 years.

Police proceeded to charge her for drug-driving, and she underwent what Ms Campbell acknowledged must have been a ''traumatic'' court process.

Evidence later emerged that the woman's husband was a dietitian, and had been making her a special gluten-free muesli. The muesli contained hemp seeds, and a medical report sought by prosecutors acknowledged the seeds could have resulted in the low level of THC found in her system.

The woman pleaded guilty to the charge on Friday, but Ms Campbell ordered no conviction be recorded and dismissed proceedings.

Rachel Bird, of legal firm Rachel Bird and Co, said outside court the laws were not a proper test of driver impairment.

Ms Bird said her client's driving was not effected in any way by the low traces of THC in her system.

She said her client had been dragged through a lengthy court process that had put enormous stress on her, and raised fears she

would lose her senior job in the public service and fail the relevant security clearances she required.

The case has also prompted criticism of the laws from Civil Liberties Australia, which has urged the government to consider a rethink.

Director Tim Vines said the random drug tests represented an invasion of a citizen's right to privacy, which was only justified if it helped protect Canberrans on the roads.

But he said that extremely low levels of a drug did not impair a driver, and did not present a threat to public safety.

''What are these laws actually trying to achieve?'' Mr Vines said.

''They're road safety laws, they're sold to us as laws that are supposed to make drivers safer, and for our roads to be safer,'' he said.

''But by adopting a zero-tolerance approach to illegal drugs, as opposed to alcohol where there's a threshold, there's no allowance for any sort of scientific basis for when a person is actually impaired.''

Attorney-General Simon Corbell, pictured, couldn't comment on the specifics of the case but said the government was not currently considering a review to the zero-tolerance approach to drug-driving.

''The current law recognises that alcohol is a legal substance and may be consumed, provided that drivers do not attempt to drive while their blood alcohol content exceeds the prescribed level,'' Mr Corbell said. ''In contrast, drugs are not permitted, and drivers should not have any content in their blood at the time they are driving,'' he said.

But Mr Vines said that was just another way of criminalising drug use, which was a completely separate issue to road safety.

''Random breath tests or any sort of random search, the invasion of privacy can only be justified if there's some public good, and the public good behind these [laws] is supposed to be safer roads,'' he said.

''So there needs to be some discussion of whether there should be a tolerance level above which you're actually an impaired driver and a risk to others,'' he said.

The woman at the centre of the case was described by Magistrate Beth Campbell as someone of good character who had contributed enormously to the community through her ''stellar'' career in the public service.

Ms Campbell said on Friday she acknowledged the defendant may have not shared her level of intellectual interest in the case. She said the court process had been punishment enough for the woman.