Disturbing picture emerges of siege gunman

Sydney siege gunman Man Haron Monis had failed to make his mark in the world and was "a man spiralling downwards" when he took 18 people hostage at gunpoint, an inquest has heard.

The self-styled Islamic sheik was broke, had few friends, was facing a possible jail term, and had failed in his ongoing quest to achieve "significance", the opening day of the inquest into the Lindt cafe siege has revealed.

And the head of the siege investigation, experienced homicide Detective Inspector Angelo Memmolo, has revealed the sheer complexity of the job, describing the Lindt cafe as "probably the most complicated crime scene that I have seen".

The picture emerging of Monis is one of an erratic, volatile but intelligent outsider with a frightening capacity for organisation.

When Monis - then called Mohammed Manteghi - arrived in Australia in 1996 he expected somehow to make a mark, counsel assisting Jeremy Gormly SC told the court.

But by December 2014, Monis apparently had no money, had failed to gain a following as an Islamic sheik and was on sexual assault charges that could put him in jail.

"It's not difficult to develop a summary of Mr Monis's life in Australia that makes him look like a man spiralling downwards," Mr Gormly told the inquest.

Extensive evidence about Monis's life, tracing back to his schooling and studies in Islam in Iran, is being presented as authorities try to understand the circumstances around the siege.

"Was Monis a so-called lone wolf prosecuting an ISIS-inspired terrorist act or was he a deranged individual pursuing some personal, private grievance?" Coroner Michael Barnes asked.

Cafe manager Tori Johnson, 34, and lawyer Katrina Dawson, 38, were killed in the siege that shut down central Sydney on December 15, 2014.

Monis was killed when police stormed the cafe in the early hours of December 16.

The inquest heard that:

- Monis tried to join the Rebels bikie gang in 2012, but bikies rejected him as "weird" and took his motorbike.

- Monis was attracted to the Rebels as a source of power and influence: "His constant goal in life appears to have been achieving significance," junior counsel assisting Sophie Callan said.

- No legal importation record can be found for the French-made Manufrance La Salle 12-gauge, pump-action sawn-off shotgun used by Monis.

- Monis enrolled in, but did not complete many courses including fitness instructing, lifesaving and criminology.

- He was convicted for writing offensive letters to the families of dead Australian soldiers.

- He had treatment for mental health problems, including depression in 2005, a form of schizophrenia in 2010, and was treated for mental health issues in 2011.

- Monis ran a "spiritual healing" and clairvoyance business between 2002 and 2007 with 500 clients, making up to $125,000 a year.

- In 2014, he was charged with 43 counts of aggravated or indecent sexual assault relating to clients of his healing business.

- In 2006, he set up a website featuring abusive messages about Australian soldiers killed in action and letters complaining to Queen Elizabeth about racism.

Security is extremely tight at the inquest, the largest to be held in NSW.

Family members of the victims and hostages of the siege are attending the hearing.

Mr Barnes said while revisiting details of the siege could be painful, a speedy investigation was critical.

"It would be unforgivable if we delayed and another similar incident were to occur before we had learned the lessons of the last," he said.

The inquest will probe Monis's employment history, and his dealings with immigration and customs officials when it continues on Tuesday.