The Australian Capital Territory (ACT) legislature is expected to pass a private member's bill to legalize the growing and possession of cannabis for personal consumption.

Labor back-bencher Michael Pettersson's legislation would allow residents of the bush capital who are 18 and older to possess 50 grams of cannabis and grow two plants.  The Greens have offered conditional support to pass it, although the opposition opposes the bill in fear of "perverse outcomes".

The ACT health minister will have to sign off on the new law, and then a review would be conducted in three years.

As far as the law clashing with the federal government, Pettersson says that Commonwealth law has a loophole that allows for cannabis use if it is excused or justified by state or territory law.

"Commonwealth law has been written with the express understanding that there are differences," Pettersson said.  "I don't think it's particularly likely the commonwealth government will try to fight this," he added.

Federal attorney-general Christian Porter said the bill was a matter for the ACT, but where commonwealth laws applied they remained enforceable.