New terror threat system starts this week

A new terrorism alert system will be rolled out this week to better warn Australians about the likelihood of an attack.

The current four levels of advice - low, medium, high and extreme - will be replaced with five tiers as recommended by the country's security and intelligence agencies.

Details of the new levels will be announced in coming days.

But the system proposed earlier this year set five categories: not expected, possible, probable, expected and certain.

It would also include an explanatory statement outlining the type of risk.

"The new system will provide the public with more information on the nature of the threat we are facing," Prime Minister Malcolm Turnbull told parliament on Tuesday.

He said it would also provide domestic spy agency ASIO with greater flexibility in determining the threat levels, reflecting the need to adapt to an "evolving security environment".

The public alert level has been at "high" since September 2014, meaning an attack is likely.