Gunman Man Haron Monis forced Lindt cafe manager Tori Johnson to kneel before shooting him once in the back of the head, an inquest has heard.

Counsel assisting the coroner, Jeremy Gormly, SC, has also revealed that barrister Katrina Dawson was struck by six fragments of police bullets after they ricocheted off hard surfaces, with one striking a major blood vessel..

The 38-year-old mother of three lost consciousness and died shortly afterwards.

The revelations came as an inquest into the deadly Martin Place siege opened at the Coroner's Court in Glebe on Thursday.

Mr Johnson and Ms Dawson were killed after Monis took 18 hostages inside the cafe on December 15.

Details of the 17-hour ordeal emerged for the first time, including a 12-minute phone call Mr Johnson made to triple-0 at 9.44am at the behest of Monis.

"Australia is under attack by Islamic State and there are a number of radio controlled bombs in Martin Place, Circular Quay and George Street," Mr Johnson told the operator.

The call was broadcast over police radio as officers from the Tactical Operations Unit and police negotiators rushed to the scene.

Monis had arrived at the cafe at 8.33am on December 15 carrying a firearm concealed in a blue plastic bag. He ordered a piece of chocolate cake and a cup of tea.

Mr Gormly said Monis asked to be moved to a different table where he could see the whole cafe and then to speak to the manager.

Other staff saw Mr Johnson becoming visibly distressed and he then told one to get the keys and lock the front door.

"We're closed," Mr Johnson reportedly said to the staff member.

"Everything is going to be OK, I want you to be calm."

Monis then stood up and declared, "This is an attack - I have a bomb."

Snipers represented

Two police snipers who killed Monis are being represented at the inquest but their identities will remain a secret.

Solicitors for the pair asked for their clients' identities to be withheld for their safety.

Monis' partner Amirah Droudis is being represented by lawyer Angelo Bilias.

State Coroner Michael Barnes opened the inquest by saying all those involved in the investigation and inquest must remain dispassionate and objective.

"We will need to rely on our reason and keep our emotions in check," he said.

"The desire for urgent answers must be weighed against the time it takes to make sure the evidence is the best available."

In his opening address on Thursday, Mr Gormly revealed that, at 2.14am on December 16, Monis shot Mr Johnson in the back of the head without warning.

The barrel of the sawn-off shotgun was 75 centimetres from Mr Johnson's head.

This was seen by a police sniper and the decision was immediately made to force entry into the cafe. The two police marksmen fired 22 shots and Monis fired two shots at police.

Monis died instantly after being hit by 11 bullets or fragments of bullets.

Mr Gormly also revealed that more than 300 triple-0 calls were made during the 16-hour siege.

He said there was no CCTV inside the Lindt cafe recording the siege but there were volumes of sound recordings plus CCTV from other buildings, Facebook pages and phone calls.

There are "hundreds of hours of material and it all has to be sifted", he said.

He said more than 30 detectives plus officers and experts were working on the case and there would be no hesitation to make adverse findings against the police if necessary.

He said the inquest would expose "any defects in the management of the siege" but it was not intended to turn into philosophical discussions or consider what "armchair hostages" would have done in the situation.

The inquest will also look at Monis' criminal history and his psychiatric health, including the charge of being an accessory to the murder of his ex-wife, Noreen Hayson Paul.

Mr Gormly said a joint federal-state government review would look at the 18 years since Monis arrived in Australia and the 10 years since he was granted Australian citizenship.

"He was on bail. It's necessary to examine without hindsight how he came to be granted bail," he said.

Mr Gormly said a lot of information would come from the hostages who have "had an experience that most of us would struggle to even imagine".

Family of some of the hostages and victims have attended the court as well as a large media contingent including international media organisations BBC, CNN, AFP and Thomson Reuters.

Security has been beefed up at the court with portable security scanners bought in to screen every person.

Thursday's opening was expected to last for two hours and was intended to be an update for the coroner. The matter will then be adjourned to a later date.