Eighteen calls were made to the national security hotline about Man Haron Monis in the days leading to the Martin Place siege but none gave any hint the self-styled spiritual preacher was about to carry out the Lindt café siege, an initial report has found.

ASIO assessed Monis as recently as December 2014, the month of the siege, but found "he fell well outside the threshold to be included in the 400 highest priority counter-terrorism investigations."

Café manager Tori Johnston and Sydney barrister Katrina Dawson were killed when the 17-hour siege ended in the early hours of 16 December, 2014. Monis was also killed in the dramatic shootout with police. In the wake of the attack, Prime Minister Tony Abbott immediately commissioned a report which he released on Sunday. It was conducted by the Secretary of Prime Minister and Cabinet Michael Thawley and the Secretary of the New South Wales Department of Premier and Cabinet Blair Comley.

It says that between 9-12 December 18 calls were made to the National Security Hotline relating to Monis' Facebook posts. ASIO, the AFP and NSW Police considered the Facebook posts "contained no indications of an imminent threat." The postings would not have been considered "advocacy of terrorism," which is a new offence federal Parliament passed late in 2014.

The report says Monis' behaviour never reached the very high threshold required to qualify for a control or preventative detention order.

Mr Abbott warned the line between individual freedoms and the safety of the community may have to be "redrawn" again and said "plainly this monster should not been in our community."

"But the report, having talked to people, having analysed everything that's on the record, says that the decisions made were reasonable enough at the time, but plainly in their totality the system has let us down.

"Plainly the system has let us down because plainly this guy shouldn't have been in the country, he shouldn't have been out on bail, he shouldn't have had a gun and he shouldn't have been radicalised to the extent that he claims to be conducting an Islamist death cult attack here in Australia," the Prime Minister said.

"So plainly we need to re-examine the system and ask ourselves at what stage do we need to change the tipping point from protection of the individual to the safety of the community and that's exactly what we're doing," he said.

The review says it understands Monis used an illegal firearm – namely a sawn-off pump-action shotgun. It says the firearm "may have entered Australia lawfully" and became a "grey market" firearm when it was not returned during the 1996 gun buyback scheme.

The report found "shortcomings in the accuracy and consistency" of firearms data and has also called for state and territory police to hold an "urgent audit" of their firearms data. The Australian Crime Commission says there are still 250,000 illegal guns in Australia.

Monis left his wife and two children in Iran and entered Australia as Mohammed Hassan Manteghi on 28 October 1996 on a business visa.

Just days later ASIO received the first of a litany of warnings about Monis that authorities would receive over the next 18 years. The information provided to ASIO about Monis on 4 November 1996 was "potentially adverse" but did not relate to a terrorist threat. ASIO responded by asking the Immigration Department to add Monis to the Movement Alert List (MAL) database.

About 700,000 people are listed on the MAL, which is used to monitor the entry and presence of people in Australia who are not citizens and considered a concern.

He sought asylum within a month and was granted a Bridging Visa in 1996, a Protection Visa in 2000 and citizenship in 2004. He changed his name to Michael Hayson Havros in 2002 and changed it again in 2006 to Man Haron Monis. He had up to 31 aliases and different spellings of his name.

The report details Monis' bizarre behaviour, including claims that he had information on September 11, the London bombings and inside information on al-Qa'ida. He began sending letters and faxes to political leaders and the media in 2007 but later that year they took on a more sinister and "provocative" edge when he began sending offensive letters to the families of Australian soldiers killed serving in Afghanistan.

In July 2008, he wrote to the Qantas CEO claiming mechanical faults in planes were the result of sabotage terrorist attacks. In November he penned letters to the families of the Bali bombers describing the bombers as martyrs.

He also mailed a DVD to the Queen warning of threats to the public and another warning of a fatwa against President Obama to US broadcaster NBC. At the end of 2008 ASIO concluded that Monis "makes sure not to cross any lines and tries to ensure he can protect himself from allegations of inciting terrorism." In March 2009 ASIO told the Secret Service that Monis "is not perceived as a terrorist threat ... but his actions may be causing offences to numerous people."

In October 2009, Monis was arrested and charged over his offensive letters to the families of Australian soldiers. (He was convicted in 2013.) INTERPOL reminded authorities in Australia that Monis was still wanted in Tehran.

Monis was interviewed by ASIO "several times" for immigration purposes, but "ultimately he was found not to be a risk to national security," the report says.

Monis was on welfare, including asylum seeker support, Newstart and Austudy, for seven and a half years of the 18 he lived in Australia.

Monis was charged in November 2013 with being an accessory to the murder of his former partner who was killed in April that year. He was granted conditional bail in December. He was charged with three sexual assault offences dating back to 2002 in April 2014 but granted conditional bail in May.

In October, he was charged with a further 37 sexual assault charges. His conditional bail was continued.

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