Australia anti-terror police charge man with being in possession of documents 'designed to facilitate a terrorist attack'

Tony Abbott said security officials had intercepted heightened level of 'terrorist chatter' in days after Sydney siege

Police patrol the street in Sydney's central business district on December 24, 2014. (AFP)

Anti-terror police in Sydney have arrested two men, charging one of the men with being in possession of documents 'designed to facilitate a terrorist attack'.

The Australian Prime Minister Tony Abbott said security officials had intercepted a heightened level of "terrorist chatter" in the eight days after the Sydney siege, but there were no specific threats of attacks.

One man, 20, was charged on Tuesday with possession of documents designed to facilitate a terrorist attack. A second 21-year-old was charged with breaching a control order, police said.

The documents had mentioned potential government targets but were not directed at the Prime Minister, Australian Federal Police Deputy Commissioner Michael Phelan told a media conference in Sydney.

Australia is on high alert for attacks by sympathisers of the radical group Isis and from home-grown militants returning from fighting in the Middle East.

Police said they had now arrested and charged 11 people with terrorism-related offences since launching massive raids in Sydney and Brisbane in September, soon after raising the terror threat to "high" for the first time.

The most recent arrests came after the monitoring of a group of 15 to 20 people who police believed may have shared an ideology similar to that of Isis.

"There is nothing that indicates at all that (there were) any specific targets or time frame in relation to this particular activity at all," Mr Phelan added.

Man Haron Monis, a self-styled sheikh who was facing numerous charges for violent crimes, held 17 hostages at gunpoint at the Lindt Chocolate Cafe in Martin Place, a central Sydney shopping and office precinct for 16 hours on Monday morning last week.

Two hostages, cafe manager Tori Johnson and lawyer Katrina Dawson, were killed along with Monis when police stormed the cafe.

Monis, who came to Australia as a refugee from Iran, complained of being tortured in prison for his political beliefs and said he was fighting for Islam and for peace.

An official investigation into the final moments of the siege and the deaths of all three is underway.

Additional reporting by agencies