Man Monis had tea and cake at Lindt cafe before taking hostages

Sydney siege gunman Man Monis calmly had a cup of tea and some chocolate cake at the cafe about half an hour before brandishing a shotgun and claiming Australia was "under attack".

The new details emerged from State Coroner Jeremy Gormly on the opening day of the inquest into the siege.

On the morning of December 15 last year, Monis entered the Lindt cafe in Martin Place, concealing a sawn-off shotgun.

He ordered a cup of tea and some cake, before he asked to speak to the manager.

Tori Johnson, who would be fatally shot by Monis just over 16 hours later, sat with Monis for a time, before giving his keys to an employee and quietly telling them to lock the doors and "remain calm".

Monis then donned a vest and headband, drew his shotgun and stood up, crying words to the effect of "this is an attack".

He then forced Mr Johnson to ring Triple Zero and tell the operator "Australia is under attack by the Islamic State" and that there were radio-controlled bombs around the city.

Monis kept the gun on Mr Johnson throughout the 12-minute call.

Mr Johnson, along with fellow siege victim Katrina Dawson - revealed today to have been killed by police ricochets as they forced entry to the cafe - was widely mourned around Australia, with a sea of flowers laid in Martin Place to mark the nation's sense of loss.