SYDNEY (AFP) - Eighteen calls were made to an Australian national security hotline about a self-styled cleric in the days before he carried out a deadly Sydney siege but none suggested an imminent attack, a review said on Sunday.

The first official government review into the incident released on Sunday said a national security hotline had received 18 calls about Man Haron Monis between Dec 9 and Dec 12, all concerned about offensive material on his Facebook page.

"None of the calls related to any intentions or statements regarding a pending attack - imminent or otherwise," the review said, adding that all were considered by intelligence and the police authorities. "On the basis of the information available at the time, he fell well outside the threshold to be included in the 400 highest priority counter-terrorism investigations," the review said. "He was only one of several thousand people of potential security concern."

Right up until the siege, intelligence and law enforcement agencies had "never found any information to indicate Monis had the intent or desire to commit a terrorist act".

Armed with a pump-action shotgun, Iranian-born Monis took 17 people hostage in Sydney's Lindt chocolate cafe hostage on Dec 15. Some 17 hours later, he shot dead cafe manager Tori Johnson, 34, prompting the police to storm the building and kill him. Another hostage died in the crossfire.

Australian Prime Minister Tony Abbott said the government was determined to learn from the review.

He said Monis, who was on bail at the time for a string of charges, had long been on the radar of security agencies, which had assessed him as being of no danger to the community.

"But plainly, in their totality, the system has let us down," Mr Abbott told reporters in Sydney in releasing the review.

"Plainly, this monster should not have been in our community. He shouldn't have been allowed into the country. He shouldn't have been out on bail. He shouldn't have been with a gun and he shouldn't have become radicalised."

In a statement, Mr Abbott and New South Wales Premier Mike Baird said the review found "there were no major failings of intelligence or process in the lead-up to the siege".

"However, the inescapable conclusion is that the system as a whole let the community down," they said.

"Monis was given the benefit of the doubt every time.

"He consistently fell beneath any threshold which would have triggered greater scrutiny or a rejection of his applications when it came to immigration, residence and citizenship, terrorism offences, national security powers and bail."

Mr Abbott said the review found that decisions were made about Monis at various levels - beginning with the Immigration Department but extending to mental health authorities, the police and intelligence agencies - which were justifiable under the circumstances.

"We don't believe that at any particular decision-making point, grievous errors were made, but the totality of decision-making let this monster loose in our community," he said.

Mr Abbott, who on Monday will deliver a national security address, said when it came to the tipping point in balancing the protection of the individual against the safety of the community, "precisely where we draw the line in the era of terrorism will have to be reconsidered and the line may have to be redrawn".

The Prime Minister said that in Monis' case, the authorities had realised fairly quickly that he had lied on his initial application to come to Australia, as he flagged more checks and scrutiny on the pathway to citizenship.

The review found that in the last months of his life, Monis had apparently become inspired by the Islamic State in Iraq and Syria (ISIS) and had rapidly become radicalised.

Australia raised its threat level to high in September and carried out a series of counter-terrorism raids following the departure of some of its nationals to Iraq and Syria to fight with ISIS and other militant groups.

"The Martin Place murderer right up to the very end was not specifically advocating violence against Australians," Mr Abbott said.

"We have some hundreds of people who are currently talking about violence against members of the community. We have many more people who are susceptible to ideologies which justify violence. We can't monitor all of them."