In his strongest public comments on the bill yet, ACT Policing's Chief Police Officer, Ray Johnson, has warned the legalisation of the drug at a territory level could even leave Australian Federal Police officers open to allegations of failing to carry out their duties.

Labor backbencher Michael Pettersson's bill would remove cannabis possession under 50g as an offence for adults under the ACT's criminal code.

However cannabis is still a controlled drug under Commonwealth law and the Constitution states that where state law is inconsistent with federal law, the latter wins.

Chief Police Officer Johnson wrote to a Legislative Assembly inquiry saying, "On the face of it, regardless of the passing of the bill, possession of cannabis would remain illegal in the ACT by effect of Commonwealth law.

"This would create a tension for ACT Policing members between their obligation to implement ACT government policy intent and to have regard for the Commonwealth criminal law," he said.

Chief Police Officer Johnson said this tension could not be addressed through an internal policy direction not to enforce the law, as such a direction would "likely be seen to impinge on the independent office of constable".

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

The Pettersson bill would also abolish the Simple Cannabis Offence Notice diversionary program set up when the ACT decriminalised cannabis in 1992.

This would mean users caught with small amounts of cannabis could face a maximum $8000 fine or up to two years in jail under the Commonwealth criminal code, instead of the current on-the-spot fine of $100 - an assessment backed up by the ACT Law Society.

"This framework is well established and built on the principles of harm minimisation and would instead be substituted by an offence with a penalty of up to two years' imprisonment," Chief Police Officer Johnson said.

Chief Police Officer Johnson also expressed concerns that allowing cannabis to be legally grown in the ACT would attract criminals to the region.

"To grow cannabis in any other jurisdiction the crop-sitters carry a significant risk of detection for the entire growth cycle," he said

"The proposed allowance of legitimate growth of cannabis in the ACT would potentially remove this risk, making the ACT more attractive to organised crime groups as a place to grow cannabis for both internal and external markets.

"Amendments proposed to allow hydroponic growth of cannabis increases the ability of criminal groups to cultivate cannabis with higher yields and resulting profits."

Chief Police Officer Johnson's submission comes as the inquiry into cannabis legalisation holds its first hearing on Tuesday.

The Families and Friends for Drug Law Reform group is among the first witnesses. While the group is in favour of the legislation, it believes the move falls short as it retains the Simple Cannabis Offence Notice for people under 18 caught with cannabis.

"An adult Canberran caught with the same quantity of cannabis or plants will face no legal consequences whereas an underage Canberran might be given an on the spot fine or even prosecuted in the Children’s Court for an offence that would not exist when he or she turned 18. This creates an unconscionable anomaly," their submission said.

They also drew on UNSW research which suggested half of the fines issued in the first seven years of the scheme went unpaid.

Families and Friends for Drug Law Reform Bill Bush suggested that the scheme should be transformed so underage drug users were referred to a special tribunal made up of drug treatment service providers, health workers and youth and social workers.

"If you think of the analogy of pill testing, the advantage is you've got an opportunity for intervention in relation to young drug users so you can get a health intervention rather than a law enforcement intervention," Mr Bush said.

However Chief Police Officer Johnson said ACT Policing had dealt with 487 cannabis offences over the past five years under the illicit drug diversion scheme  - a harm minimisation measure designed to divert drug consumers into education or treatment.

In the same period, police cleared 441 Simple Cannabis Offence Notices and gave out 40 formal cautions. Of the 209 arrests where a Simple Cannabis Offence Notice was handed out, only 10 were for cannabis alone. The rest involved charges of assault, dangerous driving, drug trafficking or breaches of court orders.