Possession of less than 50g of cannabis would be removed from the ACT's criminal code and the number of legal plants doubled under the draft legislation from Labor backbencher Michael Pettersson.

The bill has the support of the Labor executive and in-principle support from the Greens, meaning the legislation is likely to pass.

However legal experts have concerns about the conflict the legislation creates with federal laws.

Paul Edmonds - a member of the ACT Law Society's criminal law reform committee and principal of Canberra Criminal Lawyers - said while he supported the principle of the bill on a harm minimisation basis, it was inconsistent with section 308 of the Commonwealth criminal code, which outlaws the possession of controlled drugs.

Cannabis and cannabis resin are among the almost 250 drugs outlawed under this provision.

Section 109 of the Constitution also states that where a law of a state is inconsistent with laws of the Commonwealth, Commonwealth laws will prevail and the state law will be invalid.

The rule also applies to the ACT via section 28 of the Australian Capital Territory (Self-Government) Act 1988.

It's the same one that landed the ACT in the High Court in 2013, after the territory passed the nation's first law allowing same-sex couples to marry.

The experience cost the ACT more than $800,000 in legal bills.

Mr Edmonds said there was a "real possibility" that if the ACT legalised cannabis in this way, the territory was headed for another High Court challenge.

"I would hope I'm wrong on that. I don't profess to be an expert in constitutional law but prima facie, on the face if it, I can't see how it would be anything other than inconsistent," Mr Edmonds said.

He also said it may mean ACT police can still charge people with cannabis possession, but under the Commonwealth rather than territory law.

"The Commonwealth still allows states and territories to deal with criminal matters, these restrictions only deal with customs, waters, imported drugs or simply offences on Commonwealth land but legally there's no reason the Commonwealth Crimes Act can't apply to a personal in the territory who possesses cannabis," Mr Edmonds said.

"Whether the AFP would do that or have some sort of memorandum of understanding with the territory government like with pill testing where they decide not to charge people in practical terms may have the same effect but again it would seem to me o leave things in a fairly unsatisfactory and legally unclear position."

Constitutional law expert Professor Anne Twomey agreed that if the ACT law is inconsistent, then the Commonwealth law will prevail to the extent of the inconsistency.

"It is therefore possible that persons could be convicted of a federal offence, even though an ACT law treats their acts as non-criminal. However, it can be more complicated than that," Professor Twomey said.

"For example, the Commonwealth law may deal expressly with potential conflicts. See, for example section 308.1(3) of the Criminal Code, which permits a person charged with the federal offence to be 'tried, punished or otherwise dealt with as if the offence were an offence against the law of the state or territory that involved the possession or use of a controlled drug'.

"This was apparently included to allow drug users to be 'diverted from the criminal justice system to receive the same education, treatment and support that is available in relation to drug offences under state and territory laws'.

"While it is hard to see how this would work if there was no longer any offence under the ACT law, it is possible that there is some other relevant provision that would affect how a conflict is dealt with."

Asked whether they would seek to charge people under Commonwealth law if cannabis was legalised, an ACT Police spokesman said the organisation would not speculate on proposed legislation.

"Any questions related to the mechanics of possible new laws would be best directed to the ACT government," he said.

The ACT government was asked what legal advice they'd received regarding the legalisation of cannabis for personal use, and whether the territory would seek an MoU with police.

A government spokeswoman declined to say whether they'd received legal advice before Labor pledged to support the bill, but said all members of the Legislative Assembly had the right to put forward legislation for consideration.

"As is the usual course with private members’ bills, ACT government directorates are now analysing the proposed legislation, and will provide advice to Cabinet prior to the bill being debated in the Assembly," the spokeswoman said.

Attorney-General Christian Porter would need to refer the legislation to the High Court for a challenge to take place. His spokesman has previously said it was to early to determine the bill's impact on Commonwealth law.