The Federal Government has not ruled out overturning new laws passed in the ACT that legalise the use of small amounts of cannabis, which a cabinet minister has described as "crazy".

Key points: New ACT laws that allow the use of small amounts of marijuana conflict directly with Commonwealth legislation

New ACT laws that allow the use of small amounts of marijuana conflict directly with Commonwealth legislation The Commonwealth has the power to strike out ACT and Northern Territory laws

The Commonwealth has the power to strike out ACT and Northern Territory laws Federal Attorney-General Christian Porter warns Canberrans that he expects federal police to enforce federal law

The ACT Parliament passed legislation on Wednesday that allows Canberrans to possess up to 50 grams of marijuana and to grow up to four plants per household.

The laws, which are Australia's first to legalise cannabis use, are scheduled to come into effect on January 31 next year.

But federal Attorney-General Christian Porter warned Canberrans that owning any amount of the drug remained an offence under Commonwealth law, which he expected federal police to enforce.

Mr Porter also told Perth radio station 6PR the Commonwealth was investigating how it would respond to the ACT decision, suggesting it was a "social crusade".

"We'll have a look at the legislation when it's in our possession," he said, noting he was frustrated the laws had not been published as of Thursday morning.

"But, most importantly, if you're in the ACT waking up and want to possess marijuana, be careful because there are Commonwealth laws that still apply."

The ACT police, which enforces laws in Canberra, is a sub-branch of the Australian Federal Police, and reports to both the ACT and federal governments.

Commonwealth can kill off territory laws

Attorney-General Christian Porter says the ACT decision is a "very bad idea". ( ABC News: Adam Kennedy )

The Commonwealth has the power to strike out laws made by the ACT and Northern Territory parliaments. It last did so in 2006, when the Howard government overturned the ACT's decision to recognise marriage-like civil unions between same-sex couples.

Mr Porter would not be drawn on whether the Commonwealth would take this action, but he noted "there are precedents for it" and strongly criticised the ACT cannabis law.

"The ACT, by population, is a very small place: it's not much bigger than large councils … so they do go out on the edge on a lot of these sorts of social crusades, and drugs is one of them," he told 6PR.

"I must say, I think this is, personally, a very bad idea.

"But, nonetheless, they have a Parliament … I'll have a look at the actual bill and see how it actually interacts with Commonwealth law."

Constitutional law expert George Williams, of the University of NSW, said the Federal Government might opt to wait for a test case to go through the criminal courts rather than overturn the legislation directly.

He told ABC radio the Government would be unlikely to act before Prime Minister Scott Morrison returned from overseas.

"Maybe there's just no desire there or maybe … the Commonwealth is just getting ready," Professor Williams said.

"When Scott Morrison returns, they may see this as an issue upon they wish to intervene, there may be political mileage to be gained.

"So it's too early to reach a conclusion, but certainly we haven't yet seen the excitement and interest of an intervention that might be expected," he said.

However, Deputy Prime Minister Michael McCormack expressed his displeasure over the bill.

"Here's the ACT Government, probably spending too much time smoking hooch (sic) themselves, then wanting to legalise the stuff," he said.

In January, several ACT politicians opened up about their own experiences with illicit drugs.

Health risks … and health myths

Health Minister Greg Hunt also has serious concerns about the law change. ( ABC News: Matt Roberts )

Meanwhile, federal Health Minister Greg Hunt cited British psychiatric research that linked cannabis use directly to mental illnesses such as schizophrenia.

Like his colleague Mr Porter, Mr Hunt said he held serious concerns about the ACT law change, but "at this stage there are no plans to override [the ACT's laws legalising personal cannabis use]".

"It's a very significant mental health risk and I think the ACT has not taken these factors into account, and that's why one in four people who are in drug and alcohol rehab have cannabis as one of the sources of their problems," the minister said.

Additionally, Mr Porter said marijuana "has been shown to cause enormous social problems".

"To me, it's not a drug you want to make easier to access and cheaper to access; it's something that we should put every effort and resource into ensuring that people don't use — not pass laws which encourage people to use it.

"I think it's crazy," Mr Porter said.

However, the head of Canberra's peak doctors' group challenged the idea that legalising cannabis would harm the community.

Australian Medical Association ACT president Antonio Di Dio said it was important to dispel myths about marijuana, including that it was a gateway drug to using other illicit substances.

Nonetheless, he agreed that evidence had linked cannabis with depression and psychosis in young people.