"This was an atrocity - it may well have been a preventable atrocity, and that's why this swift and thorough review is so important." Prime Minister Tony Abbott told Macquarie Radio.

Fifty-year-old Iranian-born Man Haron Monis, a self-styled cleric with a lengthy criminal history, entered a cafe in central Sydney on Monday with a shotgun, taking 17 people inside hostage. The siege ended 16 hours later when police stormed the cafe. Two of the captives were killed in a barrage of gunfire, along with Monis himself.

Abbott has ordered a sweeping government investigation of the siege and the events leading up to it. Australia has seen a growing chorus of criticism in the aftermath of the attack, including why Iranian-born Man Haron Monis was out on bail and how he obtained a shotgun license.

Court documents detailing Monis' long history with the law have also begun to emerge. In 2011, Noleen Hayson Pal - his ex-partner and mother of their two sons - went to police after she said Monis threatened her life. He was subsequently charged with intending to cause fear of physical or mental harm. Pal said she feared he would carry through on his threat, noting that he'd once told her he had a gun license.

"Just like about everyone else from the premier down, I was incredulous and exasperated at this," Abbott said. "This guy has a long history of violence, a long history of mental instability, he has a long criminal record and obvious infatuation with extremism. It was extraordinary he was on our streets."

Monis was also sentenced to 300 hours of community service last year for sending what a judge called "grossly offensive" letters to families of soldiers killed in Afghanistan between 2007 and 2009.

Questions over siege handling

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 hostage Joel Herat, 21, told Fairfax Media that 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.

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

bk/rg (AFP, AP)