The gunman killed during the siege at the Lindt Chocolate Cafe in Sydney's Martin Place was an Iranian cleric with a violent criminal past.

Man Haron Monis, who was granted political asylum in Australia in 2001, was on bail for a string of violent offences, including being an accessory to the murder of his ex-wife.

He was also facing more than 50 sexual and indecent assault charges and had a conviction for sending abusive letters to families of deceased Australian soldiers.

Police negotiated with Monis, via hostages, for 16 hours on Monday before officers stormed the building at 2:00am (AEDT) on Tuesday.

He entered the cafe about 9:45am (AEDT) on Monday and held 17 staff and customers inside throughout the day.

Prime Minister Tony Abbott revealed more about Monis' background in his press briefing this morning.

What we do know is that the perpetrator was well known to State and Commonwealth authorities. He had a long history of violent crime, infatuation with extremism and mental instability. We know that he sent offensive letters to the families of Australian soldiers killed in Afghanistan and was found guilty of offences related to this. We also know that he posted graphic extremist material online. As the siege unfolded yesterday, he sought to cloak his actions with the symbolism of the ISIL death cult.

His former lawyer, Manny Conditsis, told the ABC that Monis was an isolated figure who had acted alone.

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"He came to Australia in the late 1990s as I understand it, obtained political asylum in 2001. He fled Iran because he was in fear of his life from the regime at that time," Mr Conditsis told ABC News Breakfast.

"Monis personally faced charges as a result of writing letters to the families of deceased Australian soldiers.

"They were dealt with in 2013. Subsequent to that he was charged with accessory before and after the fact in relation to the murder of his former spouse. Eventually he got bail.

"I appeared for him in December of 2013 when he obtained bail essentially based on a case that was presented by the prosecution at the time. It had significant weaknesses in it. I think that had a significant impact on him getting bail at that time."

More recently, he was charged with more than 50 allegations of indecent and sexual assault. Police allege the assaults took place in 2002, when Monis was a self-proclaimed "spiritual healer" operating out of premises in Wentworthville.

It is alleged that Monis placed ads in local newspapers offering "spiritual consultation". He claimed to be an expert in astrology, numerology, meditation and black magic.

But it was Monis' ongoing legal battle over his conviction for sending offensive letters to the families of dead Australian soldiers between 2007 and 2009 that may have tipped him over the edge.

The siege followed an unsuccessful, last-ditch attempt in the High Court to have the charges overturned.

Monis was sentenced to 300 hours of community service and placed on a two-year good behaviour bond for the "offensive and deplorable letters" sent with the help of his girlfriend Amirah Droudis.

They were sent to the families of Private Luke Worsley and Lance Corporal Jason Marks, who were killed in Afghanistan in 2007 and 2008.

He also sent a letter in 2009 to the family of the Austrade official Craig Senger, who was killed in the bombing of the Marriott Hotel in Jakarta in 2007.

Monis claimed the letters were his own version of a "flower basket" or "condolence card".

He challenged the validity of the charges in the High Court, arguing they were political.

But he lost and at trial, pleaded guilty to all 12 charges against him in August 2013.

Mr Conditsis represented Monis last year when he was charged with being accessory to the murder of ex-wife Noleen Hayson Pal, who was stabbed and set alight outside a western Sydney unit.

"Knowing he was on bail for very serious offences, knowing that while he was in custody some terrible things happened to him, I thought he may consider that he's got nothing to lose," he said.

"Hence participating in something as desperate and outrageous as this."

He said Monis claimed to have suffered poor treatment while in prison.

"He was put through let's say some very unpleasant events, involving matters of excrement over himself and his cell," he said.

Mr Conditsis said the public could be assured the siege was not the work of an organised terrorist group.

"This was a one-off random individual," he said.

"It was not a concerted terrorism event or act.

"It was a damaged-goods individual who did something outrageous."