The bill - with amendments - was passed on Wednesday afternoon with the support of Labor and the Greens.

It allows people to possess up to 50 grams of dry cannabis, or 150 grams of "fresh cannabis", and grow two plants.

Each household could grow a maximum of four plants but hydroponic growing would still be illegal.

The laws would not come into effect until January 31 and must first be signed off by ACT Health Minister, Rachel Stephen-Smith.

The legislation is an Australian first which the government says will reduce the harm of drugs and encourage people to seek help without fear of arrest.

The private member's bill was introduced late last year by Labor backbencher Michael Pettersson.

Shadow Attorney-General Jeremy Hanson said the bill was "a dog's breakfast" and people would believe it was a "free for all".

He said opposition would support some of the government's amendments, but remained fundamentally opposed to the bill.

"We do not support this legislation, which is likely to do more harm than good," he said during debate in the Assembly.

Uncertainty remains over how the bill will interact with Commonwealth laws. It is still open to police to charge people with a Commonwealth offence, which could result in prison time.

Chief Police Officer for the ACT Ray Johnson said ACT Policing would work with the government "to address the implementation" of the legislation.

Chief Minister Andrew Barr questioned whether the federal government recently intervened to get the Commonwealth Director of Public Prosecutions to change the advice it gave the territory.

He said the government received advice from the Director of Public Prosecutions on September 17, which outlined defences available if someone was charged under Commonwealth law.

But days later, the CDPP said on further consideration it would not be appropriate to provide a view on the proposed legislation.

Attorney General Christian Porter's office has denied any involvement with the CDPP's correspondence.

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Addressing how it would interact with Commonwealth law, ACT Attorney General Gordon Ramsay told ACT Parliament where there was a state and territory law, Commonwealth law allowed police and the courts to apply the laws of the state or territory.

He said by keeping the offences in the criminal code but putting an exception for people older than 18 it gave police an alternative to the Commonwealth law.

"This does not entirely remove the risk of people being arrested under Commonwealth law," he said.

He said he hoped the resources were not wasted on pursuing offences.

Despite the Greens previously welcoming the government "committing to legalise cannabis use and possession", Greens MLA Caroline Le Couteur now says it would be irresponsible for it to be reported as legalising cannabis.

She said it was actually a step to further decriminalise cannabis use.

Mr Pettersson said the majority of Canberrans supported the legalisation of cannabis and called on the Assembly to get it done.

Chief Police Officer Johnson said the force would now consider the detail of the legislation.

"ACT Policing is committed to supporting harm reduction strategies that improve the health of those addicted to all drugs, including cannabis," he said.

"It is important to note it will remain an offence to drive a motor vehicle with any level of cannabis in a driver's system and ACT Policing will continue to test for the presence of cannabis and other drugs.

"It should also be noted that it remains an offence under ACT law to supply any amount of cannabis. The offence of supplying drugs does not require the exchange of money.

"ACT Policing will continue to work with Government and partner agencies to address the implementation of the new legislative provisions for police, including the intersection with Commonwealth law."

Mr Pettersson said legalising cannabis for personal use was an important step in the drug law reform campaign.

"I am also confident that the laws passed today are compatible with current federal law due to the defence provided by federal law," he said.

"I don't think anyone should have their life ruined with a criminal conviction for possessing small amounts of cannabis."

Speaking in ACT Parliament, Mr Pettersson said some would wish the bill went further to create a market for the sale of cannabis..

"This would not be possible under current federal law, and has never been the purpose of this bill. This bill is simply about legalising cannabis for personal use."