The Australian Capital Territory law to legalise cannabis possession appears to “do nothing to end the continuing operation” of commonwealth offences, Christian Porter has warned.

The attorney general’s comments to Guardian Australia suggest Canberra cannabis users will be left in legal limbo when the laws take effect from February, contradicting the ACT government’s claim that its law provides a defence to the federal offence.

The Morrison government has stepped up its rhetoric against the laws. On Monday the health minister, Greg Hunt, accused the ACT government of being “blind and indifferent to the health consequences” of cannabis after asking it what medical evidence was considered before legalising it.

In September the ACT legislative assembly passed laws allowing adult residents to possess up to 50 grams of cannabis and grow two plants, up to a total of four plants per household.

Federal law prohibits cannabis possession but the ACT pushed ahead citing clauses which allow a defence for people engaged in conduct “justified or excused” by a state or territory law.

Porter said he was still considering the issue “on its merits” and had received a final copy of the ACT law only on Monday.

“Based on a preliminary examination of the ACT legislation, it would appear it does nothing to end the continuing operation of commonwealth laws with respect to possession of a prohibited substance, which would continue to make possession of amounts of cannabis under 50g unlawful in the ACT,” he told Guardian Australia.

“Nevertheless, I shall be considering options and will of course respond directly to the ACT attorney general in due course.”

On Monday Hunt accused the ACT government of having “no idea of the health consequences” of cannabis, warning it can have “dangerous psychotic effects” including increasing the risk of schizophrenia.

The ACT government has warned the commonwealth against interfering by passing a bill to explicitly override territory laws, as it did on euthanasia.

Hunt played down the need to do so, saying there were “already clear, strong federal laws” in place.

In the ACT the Australian federal police and its ACT policing unit enforce both federal and local laws, which could leave cannabis users at risk of arrest depending on the exercise of discretion of individual officers. An ACT policing spokesman said it was “still assessing the implications of the new legislation”.

In September the ACT government received advice from the commonwealth director of public prosecutions, Sarah McNaughton, which backed its view that local laws would provide a defence to the federal offence. She then withdrew her advice citing unspecified “legal complexities”.

A spokesman for the commonwealth director of public prosecutions told Guardian Australia the advice was withdrawn “following communications with the attorney general’s department” and “further legal consideration was given to the issues”.

The bill’s architect, Labor MLA Michael Pettersson, has dismissed concerns that legalisation would increase cannabis use, saying the law did not legalise sale or supply and arguing it was “disingenuous to pretend” cannabis was hard to access.