Commonwealth laws restricting the possession and use of cannabis would still apply in the ACT, Mr Porter told Perth radio 6PR on Thursday.

The ACT Legislative Assembly passed laws legalising the recreational use of marijuana on Wednesday night.

Mr Porter said the federal government had yet to see a final copy of the bill but would be reviewing it to see if it clashed with Commonwealth drug laws.

"I think this is a really dumb idea," Mr Porter said.

"If you're in the ACT and you're waking up today ... there are still Commonwealth laws that apply."

He said the government still expected ACT Policing, a branch of the Australian Federal Police, to do its job.

Earlier, Home Affairs Minister Peter Dutton described the new laws as unconscionable and suggested the government was considering challenging them.

"I think it might be trendy for the ACT government to go down this path, and they'll say they're enlightened and progressive and all the rest of it," Mr Dutton told 2GB radio on Thursday.

"But I think it's dangerous. Christian Porter is having a look at it at the moment."

Mr Porter has previously indicated the Commonwealth was not weighing a legal challenge.

"This is a matter for the ACT, but where Commonwealth laws apply they remain enforceable," he said earlier this week..

The laws allow residents over 18 to possess up to 50 grams and grow two plants.

Under existing laws, people with up to 50 grams or two plants for personal use face fines.

If paid within 60 days, it won't appear on someone's criminal record.

ACT's chief minister has shrugged off concerns Canberrans will be targeted by federal prosecutors when the new scheme comes into effect next January.

The territory's police have been balancing the overlap with Commonwealth for nearly three decades, Andrew Barr says.

"Does anyone seriously think the Commonwealth DPP (Director of Public Prosecutions) is going to spend all of their time, or a considerable amount of their time, prosecuting individuals in the ACT for the possession of less than 50 grams of cannabis?" he told ABC's Radio National on Thursday.

"It's one thing for police to arrest someone, it's another thing to successfully prosecute someone."

The existence of the ACT legislation is a defence if people are charged under commonwealth laws, Mr Barr says.

"My advice to everyone, is that this is an evolution not a revolution," he said.

The territory's shadow attorney-general Jeremy Hanson still thinks it's sending the wrong message, citing research showing marijuana's link to psychosis.

Mr Hanson is concerned it will lead to more drug driving and doesn't believe the laws are enough of a deterrent.

But he doesn't expect the federal government to test the overlap in the courts.

"The reality is we had a pretty good regime up until yesterday, it's not like people were being thrown into jail for cannabis use holus-bolus," he told Sky News.

Australian Associated Press