The latest bump on the road to making it legal for Canberrans to possess up to 50 grams of the drug, and to grow up to four plants for their own use, is the revelation that regardless of what is decided here Commonwealth law, which defines cannabis as a controlled substance, would take precedence.

"Possession of cannabis would remain illegal in the ACT by effect of Commonwealth law," ACT Policing chief police officer Ray Johnson said this week.

Given ACT Policing is a division of the Australian Federal Police officers would be torn between their obligation to implement ACT government policy intent and to comply with Commonwealth law.

"A failure to seize controlled drugs could leave individual police officers exposed to potential allegations that they breached the AFP Code of Conduct, allegations of impropriety, or failure to act and appropriately carry out their duties," Mr Johnson said.

Another likely unintended consequence of the bill is that local users prosecuted under Federal law would face much harsher penalties than under the status quo.

At the moment a Canberran caught with a small amount of cannabis would receive a $100 on the spot fine. If, on the other hand, they were prosecuted under Commonwealth law they could be fined up to $8,000 or jailed for two years.

Even in the event it is possible to surmount these obstacles, something which would almost certainly require changes to either the Australian constitution or Commonwealth law, other cross-jurisdictional issues remain.

Canberra's current drug driving laws are less punitive than those in NSW in that they don't impose a mandatory licence suspension. That decision is left to the court.

The statutory suspension period across the border is six months for a first offence and 12 months for a second offence.

In the event, as it seems reasonable to expect, the passage of the private members bill led to an increase in cannabis use in the ACT we would likely see an increase in the number of Canberrans driving with the drug in their system.

Cannabis, unlike alcohol, is not metabolised quickly. It can be detected by a Mobile Drug Test for up to 12 hours after use.

Given the ACT is a landlocked state nestled in southern NSW the laws in place across the border can't be easily ignored.

A Canberran who may have had a legal smoke on a Saturday night could well find themselves disqualified from driving for six or 12 months if they were stopped by NSW police on their way to visit friends in Jerrabomberra or Bungendore the following day.

NSW justifies its tough stance on the not unreasonable grounds 484 people have died in car crashes involving motorists with illicit drugs in their system between 2010 and 2017.

Given these complex cross jurisdictional issues, and the difficult position in which the passage of the bill would place local police, serious consideration should be given to advocating for a national approach to drug law reform.

There is a risk that to continue down the current path, regardless of these impediments, could do a worthy cause more harm than good.