Canberra's legal fraternity has called for a relaxing of drug-driving laws if cannabis is legalised in the ACT, saying the consequences are often too harsh and "disproportionate" to the level of impairment.

Key points: A small amount of cannabis in the system currently results in the same punishment as an alcohol reading of 0.15

A small amount of cannabis in the system currently results in the same punishment as an alcohol reading of 0.15 It is currently not possible to detect whether a person has taken a small or large amount of a drug

It is currently not possible to detect whether a person has taken a small or large amount of a drug The ACT Government has convened a working group to examine drug driving

The Law Society of the ACT made a submission to an inquiry looking into legalising cannabis, arguing the territory had the harshest drug-driving penalties in Australia.

"The penalties imposed for committing a drug-driving offence are disproportionate because they do not account for levels of cannabis impairment and intoxication," the submission said.

The society said a person who had smoked days before being stopped by police and still had cannabis in their system, but was "otherwise unimpaired", was still subject to the same penalty as repeat offender drink drivers caught with 0.08 blood alcohol levels.

But there is conjecture about how long drugs stay in a driver's system, with some reports drugs may become undetectable in roadside tests in about 12 hours.

According to the Law Society, someone who had smoked a small amount of cannabis would be subject to the same punishment as someone caught blowing a reading of 0.15 — an automatic three-year disqualification.

"Higher penalties for committing a drug-driving offence may be justifiable in circumstances where a person is impaired or intoxicated due to consuming a 'hard drug' (i.e. heroin or ecstasy), but not when a person drives with a low level of cannabis in their body," they said.

Their submission comes as the debate continues over whether the ACT should make recreational use and possession of marijuana legal.

Testing needs to be fine-tuned first: police

A drug test used by police to detect impaired drivers. ( Supplied: ACT Policing )

But the Australian Federal Police Association's president said the technology was not capable of detecting impairment from cannabis.

"At the moment the test is a positive or a negative, it's not like a drink-driving assessment where you can get a particular reading," AFPA president Angela Smith said.

"We've got a long way to go with the technology."

Ms Smith said if the technology could be developed, drug driving should be treated with low and high range tiers similar to drink driving.

"I think that is an ideal process to go to, because it takes away the doubt, it takes away the questions in court," Ms Smith said.

"[But] there's a lot of research and development that would have to go into making the technology similar to drink driving."

Road Safety Minister Shane Rattenbury said the Government would not be softening drug-driving laws with the legalisation of cannabis.

"People should not drive with drugs in their system as this endangers the community and themselves," Mr Rattenbury said in a statement.

"At this stage the Government has no intent to relax drug-driving offences."

Mr Rattenbury said the Government had convened a working group to examine drug-driving issues.