21/03/2016 11:18 AM AEDT | Updated 15/07/2016 12:51 PM AEST

'We Will Hear Evidence About What Happened During That Horrible Day'

Fairfax Media via Getty Images
SYDNEY, AUSTRALIA - DECEMBER 23: (AUSTRALIA & NEW ZEALAND OUT) A Police officer watches flowers before being removed from the Memorial of the victims of the siege in Martin Place on December 23, 2014 in Sydney, Australia. Volunteers have gathered at Martin Place this morning to begin clearing the thousands of bouquets and cards left in tribute to Tori Johnnson and Katrina Dawson, who were killed in last week's hostage siege at the Lindt Cafe. (Photo by Daniel Munoz/Fairfax Media via Getty Images)

A chilling phone call played at the Sydney siege inquest has revealed gunman Man Haron Monis forced Lindt cafe manager Tori Johnson to tell a triple-zero operator that Australia was "under attack" from Islamic State.

Johnson and lawyer Katrina Dawson both died in the 17-hour ordeal while gunman Man Haron Monis was killed when police stormed the premises early on December 16, 2014.

A third installment of the long-running inquest into the siege is underway on Monday and is looking at the siege itself, including how it was managed and responded to by law enforcement.

The inquest has heard there will be "alarming and distressing" evidence aired, including testimony from survivors of the deadly ordeal and an examination of how police responded to the events that shocked Australia.

In his opening address, counsel assisting Jeremy Gormly SC, aired CCTV footage showing Monis' approach to the cafe and sketched his initial movements inside, including ordering tea and chocolate cake while seated at table 40.

Monis, who Gormly said showed no signs of nerves in his walk to the cafe, asked to see cafe manager Johnson at 9.40am.

In the minutes after, Johnson called triple zero and was forced to read out a note hand written by the gunman, the inquest heard.

The 12-minute call was played at the inquest after state coroner Michael Barnes dismissed an earlier application to have the audio recording suppressed.

"Australia is under attack by Islamic State," Johnson says calmly on the call.

"There are three bombs in three different locations -- Martin Place, Circular Quay and George Street... I want to ask my brothers not to explode any bombs but I can’t contact because they don’t carry phones. They have radio with them. I can say that through ABC radio.

"The plan is to request Tony Abbott to call them and have a debate that is broadcast live on ABC Radio.”

Gormly said it was unclear whether Monis entered the cafe with the note, or wrote it inside.

The inquest heard that Johnson demonstrated a "calmness and command" on the triple zero phone call, setting the tone for the pair's relationship through the siege.

"He (Johnson) did it with a calmness and a command that was really quite impressive," the inquest heard.

"It demonstrated one of the features that caused Monis to treat Tori Johnson differently from others."

The inquiry has also heard that Monis changed his clothes when he got inside the cafe putting on a black headband and wristband containing arabic writing, and probably purchased the black backpack he wore in the attack in Campsie in the days prior.

It was also told that Monis withdrew $850 in the days before the siege, pretty much emptying his NAB and CBA accounts.

Upcoming evidence is likely to be tough for survivors to hear, the inquiry has also heard.

"In the coming days and weeks the people who survived the siege will give evidence about what happened during that horrible day," state coroner Michael Barnes said.

"We will also hear evidence about how the authorities responded."

He said "some of the evidence is sensitive from a security perspective and much of it may be alarming and distressing".

Barnes said he had received a report from UK police that was broadly supportive of the way NSW police handled the siege but noted it "questions some of the decision making on the day and some of the policies on the NSW police force".

He also said the inquest was still working through evidence from ASIO, with some of it yet to be finalised.

Gormly's opening address is expected to run all Monday, and perhaps into Tuesday, before the officer in charge of the coronial investigation is called to give evidence.

Witnesses to the lead up of the siege and police are also due to appear, with this section of the inquiry expected to last about 2 months.

The siege horrified Sydney and made headlines worldwide when Monis entered Martin Place's Lindt Cafe on the morning of December 15, 2014, and took 18 people hostage.

Barnes is due to hand down his findings later in 2016.

The inquest continues.