Australian PM says cafe siege 'horrific wake-up call'

Abbott has ordered an urgent enquiry into the circumstances surrounding the tragedy and why deranged self-styled Islamic cleric Man Haron Monis was not under surveillance given his history of extremism and violence.

The Iranian-born 50-year-old was on bail for a string of charges, including sexual offences and abetting the murder of his ex-wife.

"This has been a horrific wake-up call," Abbott told Macquarie Radio when asked if this was an accident waiting to happen, amid criticism that various authorities failed to act to take Monis off the streets.

"The tragedy is that this has happened. I mean, this was an atrocity, it may well have been a preventable atrocity, and that's why this swift and thorough review is so important."

Monis, who was well known to authorities but not any counter-terror watchlists, took 17 hostages at a cafe in the heart of Sydney on Monday, unfurling an Islamic flag during a 16-hour siege.

He was killed along with two victims -- the cafe's manager, 34, and a 38-year-old mother-of-three -- in a bloody end to the standoff.

Details of why police moved in when they did are sketchy but the father of a hostage who escaped said it was triggered by a group of them deciding they would "not survive until the morning if they did not do something".

Bruce Herat, the lawyer father of Joel Herat, 21, said Monis was awake and agitated and had begun herding the frightened hostages into separate groups.

"And at that point in time, Joel and five others came to the conclusion that they were not going to survive until the morning if they did not do something," he told Fairfax Media, adding that they decided to kick down an internal door.

"And I know that Joel made sure that Harriette (Denny) ... got behind him and he said, 'You're coming with me', and basically made sure that all that group were ready to go when Joel and Jarrod (Hoffman) broke down the door."

This group of people were those seen fleeing seconds before heavily-armed stormed the Lindt cafe. Police have so far not commented on the chain of events.

The siege has touched a nerve among Australians, who began laying flowers at a makeshift memorial in the heart of Sydney's financial quarter on Tuesday.

The floral tribute continues to grow bigger and New South Wales state Premier Mike Baird said a permanent memorial would likely be erected near the cafe.