Attorney-General Christian Porter has flagged possible federal intervention to overturn Canberra's decision to legalise cannabis, describing the new law as "crazy" and "a very bad idea".
He is backed by Home Affairs Minister Peter Dutton, who said Canberra's move was "unconscionable".
"It might be trendy for the ACT government to go down this path and they'll say that they're enlightened and progressive and all of the rest of it, but I think it's dangerous," Mr Dutton said, speaking on radio.
Mr Porter, who has repeatedly refused to speak with The Canberra Times on the issue, also spoke on radio on Thursday, where he said he would organise a meeting with the federal police and expected the police to enforce commonwealth law.
The ACT parliament has exempted adults from the territory laws that make cannabis use illegal, with the change to apply from the end of Janaury. But it leaves cannabis users in a legal no man's land because cannabis is still illegal under Commonwealth law.
While the federal director of public prosecutions indicated to the ACT government last week that the decision to legalise could operate consistently with federal law and that she wouldn't be prosecuting users in Canberra, she had an about-turn this week and withdrew her advice.
The police have been vague, saying decisions are in the hands of individual constables.
Asked whether police should turn a blind eye, Mr Porter said he was seeking the views of the federal police, but that was not his expectation.
"There is still Commonwealth law that applies to the ACT where a person commits an offence if they posses a prohibited substance which includes marijuana," he said.
"So whether or not this [Canberra] laws does what it seeks to do is an open question."
Canberrans should be "be careful", he said, "because there are Commonwealth laws that still apply".
Greens leader Richard Di Natale called for urgent clarification on the legal status.
"Is this just empty rhetoric or are people going to prosecuted for using can under the new scheme? We need that clarification and we need it urgently," he said.
He would be seeking said a "clear and unequivocal commitment" from government and law enforcement agencies at the coming parliamentary estimates hearings that they would not prosecute for cannabis possession in Canberra.
"It's critical that the federal government steps in and clears up the situation, and I call on the attorney-general to make it crystal clear that no one will be prosecuted under federal laws for cannabis consumption in the ACT."
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The confusion over the legalities showed the need for a national approach, Senator Di Natale said.
He called on the federal government to allow other states to follow Canberra's lead and legalise cannabis. But he said they should go further, allowing users to legally buy cannabis from regulated outlets.
Where cannabis had been legalised overseas it had reduced harms from others drugs, including reducing the supply of illicit opiates.
"And the bottom line is people are doing it anyway. Regardless of what Peter Dutton might say or what Christian Porter might say, say people are consuming cannabis."
Mr Porter dismissed the ACT parliament as "not much bigger than a large council", with a tendency to "go out on the edge on a lot of these social crusades"
"This is a terribly dangerous drug, marijuana. It destroys individual lives. The documented scientific evidence about the sustained use of it causing extreme mental health problems including psychosis is just beyond doubt.
"Why any jurisdiction would pass a law which effectively encourages more use of a drug like this is just beyond me."
The police union opposes the ACT's decision and said it would make representations to the Commonwealth.
Australian Federal Police Association president Angela Smith said, "At the end of the day, it doesn't matter what the ACT Government does, cannabis will still be a prohibited substance under Commonwealth legislation and a substance on the Commonwealth Poisons Standards. ACT Policing officers will still be able to charge someone with possession under Commonwealth law."
But Chief Minister Andrew Barr said Commonwealth agencies had "more important things to focus on than pursuing criminal sanctions against ACT residents with a small amount of cannabis in their possession".
Mr Barr's office said he didn't have time to speak to the Canberra Times directly, despite a number of other media appearances on Thursday. But he provided written comments, which continued to rely on last week's advice from Commonwealth Director of Public Prosecutions Sarah McNaughton who judged the ACT proposal consistent with federal laws, despite Ms McNaughton backtracking this week.
She initially said that anyone charged under the section of the Commonwealth criminal code that makes cannabis illegal (section 308.1) could rely on section 313.1 of the code as a defence. Section 313.1 recognised the "legitimate uses of controlled substances" and allowed conduct that was justified or excused by a state or territory law.
Ms McNaughton's advice was questioned this week by the federal Attorney-General's Department and she has now withdrawn it, telling Mr Barr in a letter on Wednesday that there were "legal complexities that we had not initially appreciated in formulating our response" and she could no longer provide a view.
But Mr Barr said the ACT Government "has not received advice from any Commonwealth agency that raises significant concerns about these reforms".
It would be highly inappropriate for federal government ministers to interfere in operational police matters, and Mr Dutton had himself acknowledged that the ACT police operated under the ACT government, Mr Barr said.
His federal Labor colleagues appear to have no interest in weighing into the debate, with Labor Leader Anthony Albanese saying the laws "are a matter for state and territory jurisdictions".
"The ACT has made a decision, but of course the federal law can also still apply in the ACT," he said.
Former federal police commissioner Mick Palmer welcomed the ACT's "courageous" decision and said other jurisdictions should follow suit.
"We need to proceed with caution, but anything that can be done to reduce the size of the uncontrolled nature of the market is an extremely positive thing to do," Mr Palmer, who has become a campaigner for drug law reform, said.
-With Daniella White