Abdul Nacer Benbrika was the first Australian convicted of leading a terror organisation and was jailed for 15 years in 2009.

He completed his sentence last year but is yet to taste freedom after Victorian Supreme Court Justice Andrew Tinney ordered he remain behind bars until November 2023 because of ongoing fears that he is a risk to the community.

Earlier this year Benbrika challenged the constitutional validity of the decision to keep him behind bars, specifically whether the Commonwealth could give Victoria's Supreme Court the power to impose the detention order.

The High Court upheld the Supreme Court's power. On Tuesday Benbrika began his Victorian Court of Appeal challenge against Justice Tinney's decision.

Justice Tinney had expressed concerns Benbrika remained at risk of future offending, but Benbrika's barrister Dan Star QC said the judge should have been specific about which offences.

He said there were references to Benbrika's desire to impart his knowledge and beliefs on others, but that was not a criminal offence of the sort that would allow him to be detained on an ongoing basis.

"There is no offence of holding extremist views. None of those offences criminalise extremist beliefs or extremist religious beliefs," he said.

Mr Star also argued the belief Benbrika was a high risk of committing a serious offence if freed was based on "inherently unreliable" notes from a deradicalisation program he participated in behind bars.

But Andrew Berger QC, representing Home Affairs Minister Karen Andrews, said it was not possible to be more precise about exactly which offence Benbrika might be most at risk of committing if he was released, adding that the law did not require the judge to be specific.

He said terrorism was a "fluid thing".

Benbrika, who is now in his 60s, came to Australia from Algeria when he was 29. He began hosting Islamic theology classes from about 2002, including for others convicted alongside him.

The self-proclaimed Islamic cleric has expressed past admiration for Osama Bin Laden and was leading a group in a plot to kill "non-believers" in an effort to convince the Australian government to withdraw troops from Iraq.

The appeal judges noted Benbrika had not harmed anybody, and nothing he had done had led to any risk of a person being harmed, prompting questions about why he is now considered an unacceptable risk to the community.

Mr Berger said the fact no harm had been done did not minimise the seriousness of Benbrika's role.

"He was leading a young group of men and encouraging them to commit very serious offences, including on the MCG, and the fact they hadn't actually succeeded at the point of being arrested is not beside the point, but it doesn't minimise the significance of it," he said.

The appeal is expected to continue on Wednesday.

Australian Associated Press