Mark Kulkens' jaw tightens and he feels a tremor in his chest, as he describes watching a city descend into crisis.

On Friday afternoon, while travelling west on Bourke Street on the 86 tram, he watched a maroon Commodore pummel hordes of people, throwing a body in the air "like a rag doll".

As the tram travelled from Elizabeth Street to William Street, he was taken on a grim "tour" of the carnage: 15 bodies on the ground in pools of blood; an upturned pram; a severely injured baby, surrounded by people in despair.

"I can feel my body reacting now as I recall it … because I go back there, I go back to the smell, the sounds, the sensations," said Mark, who is a clinical psychologist.

Since witnessing the attack, Mark has had difficulty sleeping. He has grown distracted and irritable. "The body feels hijacked a little bit."

He is among hundreds of witnesses to Friday's attack, who, days after the incident, are trying to cope with significant psychological trauma.

Director of Melbourne University's Phoenix Australia Centre for Posttraumatic Mental Health, Professor David Forbes, said witnesses can expect to experience nightmares, intrusive thoughts or memories, and to feel hyper vigilant in public, for up to two weeks after the incident.

One in five witnesses are expected to develop a mental health condition, such as post traumatic stress disorder, depression, an anxiety disorder or substance abuse problems, Professor Forbes warned.

Friday's attack involved three factors that would contribute to a witness' trauma: "One part is seeing someone else being harmed and killed. Another part is fearing for your own life and those around you at the time that it's happening. And the third part is feeling that the world you live in is no longer a safe and predictable place, and events like these can happen at any time."

Professor Forbes said those distressed by the incident should avoid resorting to alcohol or other substances to anaesthetise the pain, and to steer away from excessive media replays or reports of the event.

He advised speaking to family and friends and resuming a normal routine. A general practitioner should be sought if symptoms did not improve, he said.

Professor Jayashri Kulkarni, an expert on trauma, said some people may have delayed reactions, and symptoms can manifest more then six weeks after the event.

The Monash Alfred Psychiatry Research Centre director said it was important to watch out for symptoms in young children, which can include a fear of roads and passing cars, sleeplessness, bed-wetting and separation anxiety.

Dr Kulkarni said paying respects at the vigil in the city, and posting about the event on social media, were also valuable ways for people to express their solidarity with victims and their families.

"Partaking in these shared experiences can mean that it feels less of a burden for the individual. It can help them realise that they are not alone."

Nearly 70 volunteers from the Australian Psychological Society, Red Cross and Victorian Council of Churches were deployed daily along Bourke Street over the weekend for members of the public.

Witnesses and victims requiring support to call the Victim's Support Helpline on 1800 819 817.

For urgent support, call Lifeline on 13 11 14.

Donations for the immediate families of those killed can be made via the website www.vic.gov.au/bourkestreet.html

A vigil is being planned from 5.30pm on Monday at Federation Square.