Jailed terrorists who still pose a risk when their prison terms expire could be held indefinitely under national laws agreed to by Malcolm Turnbull and state premiers on Friday.

Mr Turnbull and state and territory leaders finalised a deal in Canberra that effectively will treat unrepentant terrorists the same as paedophiles and extreme violent offenders who, in certain cases, can already be held as a purely preventative measure after serving jail time.

The new laws, which will be drafted by the Commonwealth but adopted across all jurisdictions, are meant to address key concerns of police and security agencies about convicted terrorists from the post-9/11 era who are due to be released in coming years.

"It is an unfortunate reality that some convicted terrorists may not be rehabilitated at the end of their sentence and will continue to pose a risk to the community," Mr Turnbull said.

The Council of Australian Governments communiqué released after the meeting said the scheme would include "appropriate protections". It is understood any extended detention would require the backing of a Supreme Court judge.

Nonetheless, legal and rights advocates expressed caution about the planned laws.

Separately, the NSW government is going ahead with plans to extend to up to 14 days the length of time a terrorism suspect can be held before they are charged. Other states and territories have agreed to look at those laws with a view to adopting them, save for the ACT, which is "reserving its position".

The change to the NSW Terrorist (Police Powers) Act allows for the arrest, detention and questioning of a person if there are "reasonable grounds for suspecting that the person has committed, or is involved in planning, a terrorist act", a spokesman for Premier Mike Baird said.

A suspect could be held for four days, extended to 14 days with the approval of a judge.

Mr Baird said that combatting terrorism "requires a steady and strong action and we're responding on the back of the expert advice".

A spokeswoman for ACT Chief Minister Andrew Barr said the territory government believed "the current counter-terrorism measures are sufficient".

The plan to hold convicted terrorists beyond their jail sentences if they are deemed still to pose a risk could affect high-profile terrorists jailed in some of the most infamous terror plots, most notably the Pendennis operation that spanned Sydney and Melbourne and remains the largest terrorism bust in the country.

University of NSW law professor George Williams said post-sentence detention could be justified in cases of risk to the public, but needed to be strictly targeted and used only as a last resort.

"This is an extraordinary measure to take and can only be justified in the most exceptional cases," he said.

The person should be held only as long as they were shown to pose a risk, and the thresholds for risk should be set at a high level.

"We have to make sure there isn't just a vague risk to the community but a present, real danger," he said.

The judge should have to consider alternatives such as tracking devices, he said. Ultimately the "devil will be in the detail" as to how the laws work, he said.

Fiona McLeod, president-elect of the Law Council of Australia, said both the continued detention and the NSW proposals were "serious concerns".

She said the involvement of a judge was "a protection" but stressed that if the judge was working on the basis of information given by government agencies "there needs to be a mechanism for the review of that as soon as possible".

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