It would be "plainly open" to a police officer to charge someone with cannabis possession even if proposed laws to legalise it pass on Wednesday, the ACT Law Society says.
Labor backbencher Michael Pettersson's bill to allow adults to possess 50 grams of cannabis and grow two plants looks set to pass the ACT Assembly on Wednesday.
The government has conceded it could not eliminate the risk of prosecution under Commonwealth laws.
In a bid to stem this risk, it will introduce amendments to keep the relevant offences in the criminal code but add an exemption for adults.
Criminal lawyer and chair of the ACT Law Society's criminal law committee, Michael Kukulies-Smith, said the government's amendments went some way to reducing the risk of prosecution because police were less likely to feel obliged to follow Commonwealth law.
But it remained plainly open to individual officers to charge someone under Commonwealth laws, he said.
"If there was a complete absence of the territory law it seems to be there would be greater justification to fall back to the Commonwealth law," he said of the amendment.
"Their intention is by leaving the offence but including an exemption it sends the message to police that they are following the law when they make the determination not to prosecute."
Mr Kukulies-Smith said the government should try to get an agreement from ACT Policing it will not look to prosecute under Commonwealth law.
"It creates uncertainty where we don't seem to have a formal position from police themselves," he said.
Mr Pettersson said it would be a good idea for the government to work with ACT Policing around how officers would conduct themselves.
"But I want to reiterate that a defence does exist under Commonwealth law if the use is excused or justified by state or territory law," he said.
In question time on Tuesday, Greens MLA Caroline Le Couteur asked Attorney-General Gordon Ramsay what assistance the government would provide someone who came before court charged with possession of an amount of cannabis the bill seeks to allow.
He said if there was a prosecution in an ACT court under Commonwealth law for cannabis possession he could intervene in the proceedings.
But if a person was prosecuted under Commonwealth law in a Commonwealth court then the ACT could only intervene if a constitutional matter was raised, he said.
Among the amendments to be moved by the Greens on Wednesday is a bid to increase the amount of cannabis someone who has a prescribed medical condition may possess. They also want the bill to allow hydroponic plants to be grown. They appear unlikely to get Labor's support.
The laws, should they be passed, are not likely to come into effect until January and must be signed off on by the Health Minister.