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 Sunday.

Armed with a pump-action shotgun, Iranian-born Man Haron Monis took 17 people hostage in Sydney's Lindt chocolate cafe hostage on December 15.

Some 17 hours later he shot dead cafe manager Tori Johnson, 34, prompting police to storm the building and kill him. Another hostage died in the crossfire.

The first official government review into the incident released Sunday said a national security hotline had received 18 calls about Monis between December 9 and 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 all considered by intelligence and 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".

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," 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, 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."

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