Hostage victim Katrina Dawson was killed by ricocheting fragments of a police bullet or bullets after officers stormed the Sydney Lindt cafe siege following the execution of Tori Johnson, an inquest has heard.

The opening day of the inquest into the deaths of barrister Ms Dawson and cafe manager Mr Johnson heard graphic details of the events that unfolded at the end of the Martin Place siege last month.

"Ms Dawson was struck by six fragments of a police bullet or bullets which ricocheted from hard surfaces into her body," counsel assisting the coroner Jeremy Gormly SC said.

Mr Gormly said gunman Man Haron Monis made Mr Johnson kneel on the floor of the cafe early on December 16.

"After a short lapse of time, Mr Monis simply shot him without further notice or warning in the back of the head," he said.

"The end of the barrel was about 75cm from Mr Johnson's head at the moment of discharge.

"Mr Johnson is believed to have died immediately.

"The shot was witnessed by a police marksman who called it in.

"That resulted in immediate order to force entry of the cafe."

Mr Gormly told the inquest Monis was believed to have been trying to reload his shotgun when he was killed.

The inquest heard that on the morning of December 15, Monis went into the cafe and ordered a piece of chocolate cake.

(AAP)

He asked to be moved to a table close to the rear doors, enabling him to see the whole cafe, before requesting to speak to the manager.

Those who knew Mr Johnson could tell he was stressed by what Monis was telling him, the inquest sitting at the NSW Coroner's Court in Sydney heard on Thursday.

Mr Johnson told staff to shut the cafe down.

Monis then put on a vest and a bandanna.

He stood up, produced a pump action shotgun and by one account, he stated: "This is an attack, I have a bomb."

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