The state government will introduce what it describes as the nation's toughest terror laws, with plans to keep inmates convicted of terror offences or suspected of having been radicalised behind bars after their sentence is over if they pose a threat.

The new laws will likely have the state government work with intelligence agencies to apply to the Supreme Court for prisoners' sentences to be extended, including continued detention or supervision, for up to five years or more. Inmates charged with offences unrelated to terrorism but who are suspected of having become "radicalised" in jail and pose a credible threat could also be subject to the new provisions.

"NSW will be the first state in Australia to address this," Ms Berejiklian. "We know these are tough laws but unfortunately these circumstances are here because of what we see around the world and around Australia."

The Premier said the scheme was "drastic" but would be modelled on existing post-sentencing schemes for violent or sex offenders and would have people judged to pose a risk to community safety detained or supervised.

The Premier's comments come the day before the Council of Australian Governments, a meeting of Australian state and federal governments on Thursday where strengthened national security policies will be high on the agenda.

But the NSW Council for Civil Liberties said there was no need for any extension of post-sentencing policies.

"It can only be [a] political [measure]," said President Stephen Blanks. "It's just an abuse of what the courts are there for, which is to find truth.

"This is a regime which will result in the continued detention of people simply for what they say or think. It's fundamentally contrary to the idea of a free society". 

The Premier said her government was still considering a federal COAG proposal that would give authorities access to state government identification databases to harvest photos that could track suspects using facial recognition technology and surveillance footage. 

But Ms Berejiklian said the NSW government generally supported strengthening national security protections including through the use of technology. 

"All of us have to accept, from time to time, that our civil liberties aren't what they used to be," she said. "I'm keen to support any measure that supports public safety.

"I don't want us to say what could we have done? Sometimes it means all of us have to give up a little bit of our civil liberties.

"It's not a 'maybe' threat; the threat in NSW is probable. When the threat is probable you need to look at what you're doing."

Corrections and Counter-terrorism Minister David Elliott said state and federal agencies already monitoring inmates in prison would prepare a brief of evidence that would be presented to the Supreme Court.

Mr Elliott said "radical behaviour, letters, treatment of staff and inmates" by prisoners would inform government briefs to the courts, as would intelligence from federal and state agencies.

"There is judicial oversight," Mr Elliott said. "Not only have we got to get the evidence, the Supreme Court has to evaluate that".

The state government has already tightened bail and parole laws to make it harder for terror suspects to remain in the community and Ms Berejiklian appointed the state's first dedicated Counter Terrorism Minister this year.

"We're not looking at very large group of people here," said Corrective Services Commissioner Peter Severin. 

He said there were no impending cases of terror suspects to be released of which he was aware, but "nevertheless we need to have these laws in place so we can target those who are."

In recent years the spread of radical views by inmates convicted of terror offences is a major concern for corrections authorities.

The Premier said the government would refine its policy until November.

Existing laws for violent or sex offenders allow for orders of sentence of extensions of up to five years that can be further renewed.

About six people were currently detained and about 70 were under supervision under post-sentencing orders for other crimes, according to the state government.