The Northern Territory's testing for drug-driving started the same day a NSW magistrate found a driver not guilty of driving with an illicit substance despite a positive swab test, because the driver said he had last smoked marijuana nine days' earlier.

On Monday, when the NT introduced the testing, there were concerns from some that they may unwittingly be caught with cannabis in their system that had been smoked days or weeks earlier.

But Superintendent Bob Rennie from NT Police said at the time that testing would only record positive results to cannabis if someone had smoked the drug within the past five hours.

It is understood the technology used in the NT is the same as is used in NSW.

"Cannabis does stay in the system for several weeks, it can be detected, but judging by the amount of metabolites in the system, the police say five hours is the time in which a person would be impaired," Superintendent Rennie told 105.7 ABC Darwin.

"It is dangerous to drive under the influence of cannabis."

But in NSW the Opposition Labor Party said there was now "utter confusion" when it came to random drug testing.

The court case in Lismore came after Joseph Ross Carrall tested positive for smoking cannabis in May last year and claimed he was told by a police officer that he would pass the test if he waited at least a week after smoking the drug before driving.

But the next month he was tested again and traces of THC were again detected in his saliva.

He claimed in a Lismore court that he had not smoked marijuana for nine days and Magistrate David Heilpern said nobody was seriously contending that Mr Carrall was still affected by the drug when he was pulled over by police.

Mr Heilpern found Mr Carrall not guilty, and said the defendant had made an "honest and reasonable mistake of fact".

'Highly unusual' for positive test to active drug days after smoking

The director of the National Drug Research Institute (NDRI) at Curtin University, Professor Steve Alsop, said cannabis metabolites could stay in a person's body long after they last smoked marijuana.

"Cannabis is a fat-soluble drug and it tends to stay in our system for much longer and it can be at detectable levels up to nine, 10 or two weeks later, depending on the cut-off point that you use," Professor Alsop said.

"That doesn't mean that the person has active drug in their system, it means they have got the metabolites in their system. It doesn't mean they are intoxicated."

However Professor Alsop said roadside saliva tests usually also tested for the presence of the active drug, not just the metabolites.

He said it would be highly unusual for someone to test positive to having the active drug in their saliva days after they last ingested cannabis.

"I simply wouldn't expect it to happen however because cannabis is fat soluble, it can build up in your system and so a naive user, having one joint for example, it would be eliminated from their system reasonably quickly," he said.

"But somebody who has been using a lot over a long period of time, it would have built up in their system, so one joint would add to that background level and may be detectable for a longer period."

On Monday Police Minister Peter Chandler said the new laws aligned NT Police powers with other jurisdictions.

"If police detect the presence of drugs in your system whilst driving, your licence may be immediately suspended for 24 hours and you could be penalised with up to six months imprisonment," Mr Chandler said.