ASIO and the Australian Federal Police raised the level from "medium" to "high" in September, meaning a terrorism attack was regarded as "likely".

Federal sources said there were no immediate plans late on Monday to raise it further to "extreme".

Any further raise would indicate that authorities had specific knowledge of further plots, experts said.

Clive Williams of the Australian National University said he did not expect the level to be raised because there was no current indication of further co-ordinated plots such as a wave of attacks.

But he said there was a danger of so-called "copy cat" attacks, though he stressed it was only a possibility.

"The fear is that someone will say, 'He did something, now I'll have to do something as well.'"

Professor Williams said that the previous lift in the alert level followed authorities' concerns that spontaneous attacks might become simply too hard to predict and prevent, particularly after the Islamic State group urged followers around the world to carry out unplanned, low-tech attacks.

Greg Barton, a terrorism expert from Monash University, agreed it was unlikely there would be any immediate lift based on Monday's events.

"If they thought there was a wave of well-planned attacks coming, that would be reason to raise it further," he said. "I think it seems to be more lone wolf."

"Extreme" under the federal government's "national terrorism public alert system" indicates that a "terrorist attack is imminent or has occurred". While Monday's siege is being treated by NSW police as a terrorist incident, such an event does not automatically trigger "extreme".

Rather, it is seen in the context of the overall threat. The knife attack on two police by Melbourne teenager Numan Haider for instance did not trigger a lift to "extreme".

"Extreme is as serious as it gets – it's sort of don't go out, stay at home type of situation," Professor Barton said.