Joel Herat, 21, has told his family Monis was starting to herd the frightened hostages into separate groups inside the Lindt cafe, raising fears they would "not survive until the morning if they did not do something".

Lawyer Bruce Herat, 52-year-old father of Joel, says his son's group of hostages decided to kick down an internal door of the Lindt cafe to make their escape.

It was this decisive action, his son believes, that triggered Monis unleashing the first volley of gunfire moments later.

In an exclusive interview with Fairfax Media, Mr Herat says son Joel told him Monis was not falling asleep just before 2am on Tuesday  as previous reports have suggested, but was becoming increasingly agitated immediately before the first burst of gunfire rang out.

"From what Joel told me initially, towards 2am Monis started to bring groups of people together in close proximity from where they had been dispersed throughout [the cafe]", Mr Herat told Fairfax Media.

"And at that point in time, Joel and five others came to the conclusion that they were not going to survive until the morning if they did not do something. So I think him and Jarrod [19-year-old Jarrod Hoffman] basically came to the conclusion that they would bust down the door. And I know that Joel made sure that Harriette [Denny]  ... got behind him and he said, 'You're coming with me', and basically made sure that all that group were ready to go when Joel and Jarrod broke down the door, it was a group of six in total, I'm not sure who the others were."

This was the group that was seen on TV, making a break for safety, just seconds before heavily armed tactical assault group police stormed the cafe.

"I haven't ventured to go [deeply] into the detail of it simply because I just don't want to visit that with Joel too early," Mr Herat said.

"I don't know how they did it but they did kick down the [internal] door, which leads to the barristers' chambers, and then the moment they busted that door down, Joel heard shots in his proximity. And he made sure that Harriette got out and Jarrod got out .. there were shots as they were fleeing."

Joel did not know what was happening to the other groups in the restaurant at that point.

"It had to be immediate, no opportunity to look back or anything like that, they just had to take their opportunity," his father said.

Mr Herat says families of the hostages had been waiting in a special room set up by police in the old courts for several hours, getting regular briefings from police, but that for operational reasons police had decided there should be no TV in the room where they were waiting.

Immediately after Joel's group made their escape, Mr Herat says, "all hell broke loose" as special police units stormed the cafe.

The families could hear the commotion and "everybody in that room went to water.. everybody surged towards the street" but were initially held back.

"Everybody in that room just let out an awful sigh and a bit of a wail, you could see it on their faces. It sounded like instant fire but what it was was the flash grenades that were going off initially and then moments after that we knew there was bullet fire and it went on for a long, long time."

Mr Herat said there was confusion for a while immediately after the assault and a wait before he knew Joel was absolutely safe.

He was not reunited with his son until about 6am or 7am, and was overwhelmed to be able to embrace him again.

But his heart is heavy, he says, for the two families whose loved ones - manager Tori Johnson and barrister Katrina Dawson - did not make it out.

He said those families were taken aside to be given the news, but the others could hear their cries of anguish.

"It was the saddest thing I have ever heard," he told Fairfax Media.

Earlier in the day, Monis had grown extremely agitated after the first group of hostages escaped about 4.30pm on Monday.

"We knew that three escaped early on, and I knew that was fabulous, but we were very much worried that some form of retribution would be taken. Luckily it wasn't but I know that when I spoke to Joel later he said that upon the first escape, [Monis] lined up five people, one of them was Joel, and pointed a gun at them and said, 'Someone must die now'."

After that threat, Herat said: "I think they talked him down. It progressed to another level, in terms of them being asked to do things, put the flag up again, being positioned in certain areas where he could monitor and control them. And beyond that I don't know."

The first sign that something had gone terribly wrong with his son's day came when a friend of Joel's texted his father sometime before 10am on Monday, as Mr Herat was walking to work, to ask if Joel was working at Lindt that day.

"I was about to reply, yes, drop in and have coffee with him.... and I asked why, and he said, "I think you had better get up there, there is a situation happening."

"I just ran" Mr Herat recalls. "I ran from Chippendale to Martin Place."

For the first hour he was kept at police barricades without information, and then walked around to Elizabeth Street, where he was told an interim family room was being set up in a nearby club.

"About two and a half hours later we were told that a special room was being set up [for the families] in the old criminal court house" Mr Herat said.

"We were given very neutral information first at around 2pm on Monday. Looking back it was frustrating ... but they wanted to be able to deliver information efficiently, when all the families were gathered round, rather than having to repeat it a dozen times when each of the families arrived, so I can see the logic of it."

He says he has "nothing but praise for the New South Wales police".

The atmosphere inside the family room was  subdued, but calm, Mr Herat says.  Numbers fluctuated through the night, some leaving as others arrived. Food and water was brought and police supplied regular updates, asking that if the hostages contacted family members  via social media they should let authorities know immediately.

"We all had access to smart phones which enabled us to get information from the net," Mr Herat recalls.

"Joel was forced to upload the videos [of other hostages' pleas] through his Facebook and YouTube account.  I felt somewhat cold when I saw those., thinking if [the gunman] is going to that resort, then obviously he is starting to target people as emotional conduits to get what he wanted. My concern then was how far would he go."

Monis was highly controlling, but also erratic in his moods, Joel reported later.

"He was trying to manipulate them, being nice one moment, authoritarian the next, or trying to explain that his were reasonable requests. He was trying [at times] to win the hostages over."

Joel said Monis reacted sharply to noises. One that particularly set him on edge was the periodic churning of the ice machine.  "Every time that happened every hour he would turn around and say, 'someone is trying to break in - someone must die.' "

'There was no obvious signs of him closing his eyes or falling asleep.".

Recalling the moment when he knew his son was safe, Mr Herat describes feeling "utter relief and absolute stress just falling off you."

But then came the devastating knowledge that Tori Johnson and Katrina Dawson had not made it out alive.

"It was a bittersweet moment because we were absolutely shattered that their families could not be reunited as well. We were joyous for our son, but stricken with sadness for the others".

As police broke the news to the bereaved families, "it was the saddest thing I've ever seen and heard in my life".

Mr Herat did not tell Joel about Tori Johnson's death until they reached home on Tuesday morning.

"He fell to water; he and Tori were great friends."

Mr Herat says he is " enormously proud" of Joel, who is being supported by family and surrounded by friends. Joel received initial counselling after the siege was over. "At this stage I don't notice anything unusual. I'll be more worried about those quieter moments, when reflection will set in."