"My view sitting here today is that I have no absolutely concluded view," Sarah McNaughton told estimates hearings on Tuesday, saying she would wait until police brought a brief of evidence to decide.

Cannabis users are now facing a legal quagmire. The ACT Assembly has legalised cannabis from the end of January. But the Commonwealth government says cannabis possession remains illegal under federal law and it expects the police to prosecute. The ACT Police have previously said that because the new laws end the decriminalisation regime, they could leave users facing fines of up to $8000 or two years' jail.

ACT Liberal spokesman Jeremy Hanson said a sensible system had been replaced with "an incompetent, incomplete law which opens Canberrans to much harsher penalties than if they had just left the system alone".

The ACT government has effectively challenged the federal authorities to bring a test case.

"If Commonwealth agencies ... prioritise the prosecution of Canberrans caught with a small amount of cannabis, then that is a matter for them and for the federal attorney-general to defend," a spokesperson said on Sunday. "If more people are going to be incarcerated under these laws, then the conservative Liberals better have a plan for how they are going build more prisons."

The ball is now in the court of police, which have so far not said what they plan to do if someone is caught with cannabis.

Ms McNaughton said she would decide whether to prosecute when and if she was handed a brief of evidence for a prosecution.

"It would depend what the law looked like when and if I received a brief," she said.

On September 17, she told the ACT government their plan to legalise cannabis was consistent with federal law. The commonwealth criminal code allows conduct that is "justified or excused by a state or territory law", and Ms McNaughton originally said that anyone charged under Commonwealth law could use that section as a defence, so her office wouldn't prosecute. But a week later she backtracked, citing unspecified "legal complexities".

Mr Porter said on Sunday section 313.1 could only be used as a defence if the ACT had created "a positive right" in law to use cannabis - which it had not.

Ms McNaughton has now named the tax case that Mr Porter and his department are relying on - Denlay vs the Commissioner of Taxation. She did not detail the legal argument, but said the tax case suggested the excuse under state law had to be a positive one, and it was clearly open to the Attorney-General's Department to come to the view that cannabis remained illegal.

Ms McNaughton told the Senate estimates hearings that she had been contacted by the Attorney-General's Department after her September 17 letter, alerting her to the Denlay case.

"I became aware of that case and on becoming aware of that case that's when I decided that the previous view was no longer one that I held," she said. "... It was enough that it gave us pause."

She had decided "it wasn't an appropriate use of our resources to provide a concluded view", she told the hearing.

"It is a more complicated issue than first appears and should a brief come we should wait until that happens to see what the legislation might look like at that date."