If the Morrison government moves to overturn the legalisation of cannabis in the territory, it will need four of six crossbench votes. While none have declared their hand outright, the mood is not sympathetic.

ACT Chief Minister Andrew Barr is preparing to make the argument to the crossbench, but said any intervention would be "a massive overreaction".

Laws passed in the ACT Assembly on Wednesday make it legal to possess 50 grams of cannabis and grow two plants a person from January 31, 2020.

Senator Rex Patrick, whose Centre Alliance has two crossbench votes, said if a bill to came to the Senate, he would seek medical, law enforcement and other advice, perhaps through a Senate inquiry.

He was "open to the argument", but his "starting point would be to say no", given the harms of drug use. Drugs were not like euthanasia, which was an issue of conscience where states and territories should have the right to make their own law, he said.

With Senator Patrick already expressing doubts, the odds are stacked against the ACT laws lasting long enough to take effect on January 31.

Jacqui Lambie would not comment. Previously, she has said she does not support drug decriminalisation, but she is sympathetic to users. Cory Bernardi also declined to comment.

Senior Coalition ministers lined up to condemn Canberra's move, describing it as "crazy", "dangerous", "unconscionable" and "madness".

"Here's the ACT government probably spending too much time smoking hooch themselves, then wanting to legalise the stuff," Deputy Prime Minister Michael McCormack said.

Prime Minister Scott Morrison said the government would consider the laws carefully before they took effect in January and before making a decision.

Mr Barr accused the Coalition of becoming "obsessed with small-scale personal use within the ACT" .

"They will beat their chest as they want to do but they've got bigger issues to worry about I would have thought."

Mr Barr said it would not be easy for the Coalition government to overturn the laws, requiring the support of the Senate.

"It's a difficult path for them, it's not as straightforward as it used to be," he said.

Asked whether there were plans to decriminalise other drugs such as MDMA, he said further laws could be on the agenda but not within four to five years.

"I wouldn't suggest anyone hold their breath on anything else at this point," Mr Barr said.

The Commonwealth is also likely to push for a hard line by federal police to enforce the federal law that makes cannabis possession illegal nationwide.

That leaves users in a legal peril and potentially facing serious penalties. The federal criminal code allows up to two years' jail or a $84,000 fine for possession.

But the legal situation is debated. Even the Commonwealth director of prosecutions seems unsure, initially telling the ACT government its law was consistent with federal law (because the federal Criminal Code that bans possession also contains an exemption if something is allowed under state or territory law), but then withdrawing her advice after talking to the attorney-general's department, which argues that the exemption doesn't necessarily apply.

Constitutional lawyer George Williams said the DPP's about-turn and the shifting advice was "certainly irregular".

"You can see where DPP started. You can see what the Attorney-General's department has done. And in the end it may need to be tested in court. It's an extremely complex interaction between two laws and the only way to know for sure is if someone is prosecuted," he said.

Professor in criminal law at Melbourne Law School Jeremy Gans said it was "very unclear" whether a successful defence could be mounted if the police decided to prosecute someone for possession under federal law.

The criminal code exemption was vague, and it had been designed to allow states and territories to licence activities such as providing medical cannabis or growing hemp, not drug use.

The upshot, according to Prof Gans, is that Canberrans wanting to smoke marijuana must still worry about the federal ban.

The situation in Canberra was made trickier by the fact that Canberra doesn't have its own police force, unlike the states, he said.

If the Commonwealth wanted to overturn the law it would have to do so through the parliament rather than a High Court challenge, as it did on same-sex marriage. On marriage, the Commonwealth argued successfully that the ACT law was inconsistent with the federal law, but Prof Gans said there was no credible argument that the cannabis laws were inconsistent, since the federal government could criminalise whatever it wanted to, and the ACT could choose to criminalise less if it wanted to - as in this case.