As advocates prepare to mark the first day of the ACT's nation-first laws, the police union is calling for "urgent clarification" on how officers should deal with people caught in possession of the drug.

Australian Federal Police Association president Angela Smith said officers were confused about whether they should be enforcing territory or commonwealth law.

The confusion stems from a political dispute between the Barr government and the federal government over the legal effect of the territory's cannabis laws, which passed the ACT Legislative Assembly in September.

The laws allow adults to possess up to 50 grams of cannabis and grow two plants, with a total of four plants per household permitted. Smoking in public, or near a child, will be also banned.

However, cannabis possession is still prohibited under federal law.

The ACT government has maintained that the legislation passed last year will provide Canberrans with a defence against those laws.

But federal Attorney-General Christian Porter said his legal advice had determined that the defence wouldn't apply, and the ACT's cannabis laws "had not done what they [the ACT government] think it does".

Mr Porter has made it clear that Commonwealth laws should be enforced.

Speaking just hours before the new regime started, Ms Smith said the uncertainty hadn't been cleared up for the police officers who would be expected to enforce the laws.

"Our big concern is how are officers supposed to police this?" she said. "That issue is still outstanding and we want and need urgent clarification."

Chief Police Officer Ray Johnson told The Canberra Times that the view of his organisation was that federal laws still applied in the ACT.

But he said it was ultimately up to individual officers to decide which laws to enforce. He said his message to police officers was to focus on "harm minimisation".

Chief Police Officer Johnson was confident that junior officers would be able to make "good, solid decisions" when dealing with a case of cannabis possession. He said training was provided to officers.

He expected that any confusion surrounding the enforcement of the laws would be cleared up in the next six months.

"Probably in six month's time, it won't be an issue," he said. "It will be clearer on how it works, and how our people make it work."

ACT Attorney-General Gordon Ramsay said the ACT's laws had been carefully drafted so that they could sit "comfortably side-by-side" the commonwealth laws.

"We believe that the laws are clear and policing have the clear capacity to be able to operate within the two pieces of legislation," Mr Ramsay said.

Mr Ramsay said he would be "disappointed" if the federal government sought to intervene in any way, including by directing police to enforce commonwealth laws.

He said the ACT government would be prepared to defend the laws in court.

"If there were to be the case that the commonwealth decided to put resources into prosecuting a (a case involving) a small piece of cannabis being used by someone .... then I would be prepared to offer our assistance, as it is appropriate in the circumstances," he said.

Labor backbencher Michael Pettersson, who introduced the cannabis bill to the ACT Legislative Assembly in September 2018, described the commencement of the new regime as a "historic day".

"It's been a long time coming and builds on previous progressive cannabis reform in the ACT," he said.

"I am proud of Canberra for once again leading the nation in sensible drug law reform. The legalisation of cannabis seeks to reduce the harms experienced by cannabis users and divert them away from the criminal justice system towards health and support services."