It comes after the revelation that Monis may have initially planned to attack the offices of Channel Seven in a terrorist attack.
In 2009, Channel Seven's Today Tonight show exposed Monis for sending abusive 'condolence' letters to the families of Australian soldiers killed in Afghanistan.
The program, which labelled Monis a 'Fake Sheik', led to the conviction of he and wife Amirah Droudis in the High Court over the letters.
On several occasions after the program aired, Monis protested on the steps outside the Seven Studio with pro-Islamic leaflets.
In the early hours of Monday morning, it's understood the gunman walked around Martin Place before entering the Lindt Cafe and sitting for up to an hour before taking customers and staff hostage.
One of Monis' former lawyers encountered the man in the cafe by chance on Monday morning and described his behaviour as distinctively calm.
"He was cool as a cucumber, not agitated, not scoping the place out, not looking over his shoulder," said the lawyer. "He kept asking, 'Why are you here?'. I think he was genuinely surprised to see me there."
The evidence comes after reports that the siege gunman told police in 2008 he planned terrorism-related "protests" in Martin Place just metres from the café where he took 18 hostages this week.
Man Haron Monis also wrote letters to Qantas's chief executive in 2009 suggesting he had information about sabotage at the airline that would lead to mass casualties.
Prime Minister Tony Abbott has launched a major investigation into why security services allowed Monis to drop off a terrorism watch list despite numerous examples of extremist views and behaviour, reports Sydney Morning Herald.
In one post on his website, Monis writes about a meeting alleges he had with New South Wales police in 2008 in which he said one of the detectives asked him about a protest outside the Seven Network studios in Martin Place against the way the network's program Sunrise had reported a terrorist attack overseas.
The protest was carried out by Monis' wife, Amirah Doudis.
According to the post, Monis was asked by police if he planned further protests in Martin Place, to which he replied: 'Well, it depends if the Australian government condemns the terrorist comment which was broadcast by Sunrise (then) I don't have to continue but if the Attorney-General doesn't condemn Channel 7 for its terrorist act I believe it is my religious duty to continue.'
He wrote that he also told police during the discussion: "If our fair defence against your unfair attack is called terrorism we are proud to be called terrorists."
The Seven studio is immediately opposite the Lindt café on Martin Place that Monis raided this week.
A separate post in August 2008 revealed Monis writings to then-Qantas chief executive office Geoff Dixon.
Monis wrote to Mr Dixon: "I have sent you a letter warning dated July 28 about upcoming technical problems which will be caused deliberately by some people within Qantas. .. if you do not take my warning seriously Qantas will witness similar technical problems. .. if you keep ignoring my offer for sharing information you will be responsible for the death or injury of people.."
Qantas reportedly referred Monis' letters to Australian Federal Police at the time.
Monis was also on bail for a long list of charges, including sexual offences, conspiracy to murder his ex-wife and a series of abhorrent letters.
In September 2013, Monis was sentenced to 300 hours community service for a series of letters he sent to the families of seven Australian soldiers killed between 2007 and 2009.
In the letters, Monis likened soldiers to murderers and killers, and in one letter said the man was going to hell.
Monis was also on bail on a charge of conspiring with his girlfriend, Amirah Droudis, who is allegedly a Muslim convert, to murder Noleen Hayson Pal.
Ms Pal was lured to the Werrington apartment block in western Sydney and allegedly doused with accelerant, set alight and then stabbed 18 times by Droudis.
Monis and Amirah Droudis were arrested on November 15 last year and charged.
Monis also had outstanding charges over allegations of indecent and sexual assault relating to his career as "a spiritual healer".
He was due in court due in court in February next year for the sexual assault charges, which included 22 counts of aggravated sexual assault and 14 counts of aggravated indecent assault against seven women, including one 27-year-old who answered his ads placed in ethnic Sydney newspapers.
Speaking to ABC, Prime Minister Tony Abbott questioned why Moni- who was well known to both federal police and ASIO - was not on any counter-terrorism watch lists.
"These are the questions that we'll be having our officials ponder because he was a person who had been of interest to our security agencies," Mr Abbott said.
"We want to know why he wasn't being monitored, given his history of violence, his history of mental instability, and his history of infatuation with extremism."
The Prime Minister said Monis was inadequately dealt with, and that the events of the siege should serve as a grave learning lesson.
He announced an urgent review to examine why Monis had access to firearms and why he was granted citizenship in Australia.
