Mr Barr told an ACT parliamentary inquiry on Wednesday that the legislation before the ACT Assembly to legalise cannabis for personal use would not change drug-driving laws. But he pointed to the fact that cannabis can be detected days after smoking. The question of when someone was safe to drive would differ from person to person, as with alcohol, he said.

Mr Barr compared the issue to taking cold and flu tablets and antihistamines that make people drowsy and might impair driving.

"We're not as obsessed about that as we appear to be on this question, but it is obviously a factor and government will have to look at that," he said, answering questions from Labor colleague Bec Cody, who pointed to a drug-driving regime in Canada that measures levels of impairment.

The government was "trying to make it work", not to find "100 reasons" why it couldn't work, given the current regime was far from perfect. Ideally, cannabis would be legalised, taxed and regulated, rather than the model set out in Labor backbencher Michael Pettersson's legislation being considered by the ACT parliament. That would allow greater assurance about what was being sold and consumed, and was "the ideal outcome", he said. But the ACT did not have the power to go that route and was doing what was possible.

While Labor has proposed amendments to Mr Pettersson's legislation, limiting plants to two per person instead of four, and a household limit of four, Mr Barr agreed questions remained, including how much cannabis would constitute supply rather than personal use, how dry and wet weights would differ, and whether a proposed ban within 20 metres of a child was workable in apartment buildings.

It might not be possible to address all the issues, but his starting point was to find a way to make it work "rather than adopt the Nancy Reagan approach of just saying no".

Teenagers were already consuming cannabis, he said, questioned from Liberal Assembly member Vicki Dunne about whether legalising cannabis would encourage children to smoke. There were competing views in the medical profession and community about the extent of the risk, and already "a lot of harm caused by overeating" and drinking too much.

"Everything in moderation seems to work for most people as a guiding life principle, don't do anything to extreme ... and that's also how one might approach dealing with legislation like this..." he said.

"There is no perfect answer here, you have to accept that there are risks involved in any legal regime, any health intervention regime, and at some point you've got to make a value judgement, a moral judgement ...

"The war on drugs is not successful in stopping people from using illicit or illegal drugs, it's just simply lining the pockets of cartel suppliers. That is an inescapable fact."

ACT Solicitor-General Peter Garrisson also appeared, questioned on whether the ACT could to legalise cannabis in the face of Commonwealth legislation. Mr Garrison said the critical issue was whether the final form of the ACT legislation was not so "wildly divergent" from the commonwealth law that it was incompatible. That would depend on careful drafting and there was "continuing dialogue" in the ACT justice officials about the amendments that might be needed to make it work, he said. Ultimately, it was a matter for the courts.