A coronial inquest into the December Sydney siege has heard how gunman Man Haron Monis ordered tea and cake before producing a shotgun and taking over the Lindt Cafe.

Speaking at the opening of an inquest into the siege, Counsel assisting the coroner, Jeremy Gormly SC, revealed how Monis executed cafe manager Tori Johnson with a shot to the back of the head, causing police to storm the building.

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Mr Gormly also confirmed speculation Katrina Dawson was struck by fragments of police bullets during the siege’s dramatic conclusion, causing her to lose consciousness and die shortly afterward.

The inquest heard how Monis shot Johnson in the back of the head without warning at 2.14am, causing tactical police to storm the cafe just seconds later.

Victims Katrina Dawson and Tori Johnson.

“Mr Johnson was made by Mr Monis to kneel on the cafe floor, and after a short lapse of time, simply shot him without warning in the back of the head,” Mr Gormly said.

“Mr Johnson is believed to have died immediately.”

“The shot was witnessed by a police marksman who made the immediate order to force entry to the cafe.”

Mr Monis entered the cafe at 8.30am on the 15th of December, ordering tea and chocolate cake, before asking to be moved to a table which had a full view of the cafe.

Around 30 minutes later, he asked a waitress if he could speak to the manager of the cafe, Tori Johnson, and after sitting with Mr Monis, Mr Johnson became “visibly stressed”.

“Mr Johnson said to another employee something like ‘I need you to get my keys from the office and lock the doors, we’re closed. Everything is okay, tell the staff to be calm’,” Mr Gormly said.

The court heard how Mr Monis claimed Australia “was under attack by the Islamic State” after taking over the cafe, and said he had remote controlled bombs in several CBD locations – a claim which proved to be false.

Mr Monis also lied about having a bomb in the backpack which he wore throughout the duration of the siege.

Mr Monis was killed by police fire while attempting to reload his shotgun.

He was struck twice in the head by fragments of police bullets, and 11 times in the body.

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The psychiatric state of Mr Monis and the conduct of police will be thoroughly investigated by New South Wales state coroner Michael Barnes as part of the inquest.

In his opening address, Mr Barnes said he was being assisted by a “crack team of detectives” and would do everything within his power to establish what happened.

The siege prompted an outpouring of grief from Sydneysiders, who left floral tributes en masse in Martin Place in memory of the victims.

A number of survivors have given paid-for interviews about their experiences during the 18-hour ordeal.

The inquest has now been adjourned for a later date.

Meanwhile, British police experts have been called on to provide an independent review of the Sydney siege police operation, which was announced at the inquest.

Along with domestic and internal reviews, an assessment will be undertaken by experts from the Office of the Chief Coroner of the United Kingdom.

“The purpose of that course is to have an assessment independent of NSW Police,” counsel assisting the coroner, Jeremy Gormly SC, said.

“(They) have particular expertise in siege management.”

The team will include a few interstate Australian policing experts for domestic context.

Siege timeline – what we know:

December 15, 8.33am

Man Haron Monis enters the Martin Place Lindt Cafe and orders tea and a slice of chocolate cake.

He then asks waitress if he can move to a table to the rear of the cafe, giving him a full view of the building.

Monis asks to speak to the manager of cafe, Tori Johnson, who becomes ‘visibly upset’ while speaking to Monis.

Johnson tells employee to lock the doors, saying cafe is closed.

Police storm of the cafe. Photo: AAP

9.44am

Johnson places 000 call at request of Monis, telling operator Monis has pointed gun at him and that he and others in cafe have been taken hostage.

Monis says Australia is “under attack by the Islamic State” and claims to have placed radio controlled bombs around the Sydney CBD.

Hostages are made to place an “Islamic style flag” in window of cafe, and police declare the incident a possible terrorist siege.

3.37pm

Lawyer Stefan Balafoutis and 82-year-old John O’Brien are the first hostages to escape.

5.00pm

Two Lindt Cafe waitresses escape and are taken to safety by tactical response police.

December 16, 2.07am

Another group of hostages, including more wait staff, escape the cafe.

2.14am

Monis asks Johnson to kneel on the floor of the cafe, before shooting him in the back of the head with “no warning”.

A police marksman sees Monis shooting Johnson, and tactical response police storm the building seconds later.

Monis is killed by fragments of police bullets while attempting to reload his shotgun.

Lawyer Katrina Dawson is struck by fragments of police bullets, causing her to lose consciousness and die shortly afterward.