Australia's top security advisers say they can't rule out a large coordinated terrorist attack inspired by events overseas, but a lone wolf is more likely.
The government on Thursday unveiled an updated terrorism threat advisory system, but did not raise the threat level which has been in place since September 2014.
The new system will have five tiers, instead of four, using the terms not expected, possible, probable, expected and certain.
The "high" level set in September under the old system will translate to "probable".
Also released along with the changed alert system was the first "statement of public advice" on the terrorism threat.
"The most likely form for a terrorist attack in Australia would be an attack by an individual or a small group of like-minded individuals," the statement said.
"However, a larger, more coordinated attack cannot be ruled out."
The statement said credible intelligence indicated "individuals or groups have developed both the intent and capability to conduct a terrorist attack in Australia".
Prime Minister Malcolm Turnbull said the government would be "calm, cool and clear-eyed" about using the best expertise and technology to protect the public against terrorism.
"We cannot eliminate entirely the risk of a terrorist incident any more than we can eliminate the risk of any other crime," Mr Turnbull told the ABC's 730 program.
ASIO chief Duncan Lewis told reporters in Canberra it was important for the public to know that the alert level had not been elevated.
"We have no evidence to suggest a Paris-type attack is imminent here at all," he said.
"But we have seen tragically three now lone-actor events in this country."
Security officials fear a small number of Australia-based ISIL sympathisers and supporters might be emboldened by the perceived success of their overseas counterparts.
"Elements of some of these recent attacks, such as the use of firearms and explosives as weapons, the capturing of hostages, and the focus on 'soft' targets, could be employed in an attack in Australia," the terror advice statement said.
Attorney-General George Brandis said the change in language would make it easier for the public to understand the threat level.
The level will be set by the ASIO chief on expert advice from his officers.
Mr Lewis said any specific information about an attack would be communicated to the public "as quickly as we possibly can", so protective action could be taken.
Australian Federal Police chief Andrew Colvin said the public should remain vigilant but go about their lives as usual.
He said under the new system police in each state and territory would provide the public with local information about the security environment.
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