Police have named the man they are hunting over the fatal shooting in central Melbourne.
He is Christopher Wayne Hudson, 29, a Victoria Police spokeswoman said in a statement.
The Nine Network reported that Hudson was a member of the Hell's Angels motorcycle gang and he was shot during a violent brawl on Queensland's Gold Coast last year.
Last year, the Southport Magistrates Court was told Hudson had been shot twice - once in the jaw and once in the back - during a brawl at a kickboxing event.
It was claimed the shooting was retribution for defecting from the Finks to the Hell's Angels bikie gang.
The brawl occurred at a kickboxing tournament at the Royal Pines Resort at Carrara in March last year.
The police spokeswoman urged anyone who saw him to contact police or Crimestoppers.
"Under no circumstances should anyone approach Mr Hudson," she said.
One man died and two others were seriously injured when a man opened fire at the corner of William Street and Flinders Lane, as hundreds of people headed for work in the morning peak.
A 43-year-old solicitor died at the scene, despite the efforts of paramedics to revive him.
An earlier altercation at a nightclub turned into a violent confrontation on the corner of Flinders Lane and William Street at 8.15am (AEST), when two good Samaritans went to the aid of a woman being dragged from a taxi by her hair.
Without warning, her attacker pulled out a handgun and, at point blank range, blasted the two men in the upper body before shooting the woman.
Paramedics tried in vain for an hour to save the life of the solicitor.
The second bystander, a man in his 30s, is in a critical condition with gunshot wounds to the upper body.
Detective Inspector Stephen Clark said the dead solicitor's last act "was one of extreme bravery".
"It's a tragic, tragic set of circumstances and it does appear as though he's been in the wrong place at the wrong time and has gone to the assistance of a female.
"It really strikes at the heart of all investigators to see somebody who's going about their daily life to be killed in such a tragic way," he said.
The woman, believed to be a 24-year-old travel agent, suffered serious upper body gunshot wounds and is in a serious but stable condition.
Both the wounded rushed to Royal Melbourne Hospital, where the woman's mother told news.com.au: "My daughter is up there fighting for her life."
She added: "I just feel so sorry for the other family."
Eyewitness Zali Nash said the shooter was "as cool as a cucumber" as he shot his victims before fleeing on foot.
"He just went bang, bang, bang, there was no mucking around," Ms Nash said.
"There were five shots fired and the people went straight down to the ground. There wasn't even a scream, just a whimper."
Despite an extensive search of the area, which was cordoned off by police, the gunman remains at large.
Eyewitness Ross Murchie said the shooter pulled his gun out after the two men tried to help the woman.
"She was screaming and a guy had her by the hair," Mr Murchie told ABC Radio.
"She tried to grab hold of a taxi that was going by and the couple of bystanders went over to ask what was happening.
"He let go of her hair, pulled out a gun and shot them all."
Police appealed to the taxi driver, and other witnesses, to come forward.
Interstate authorities have been alerted to the suspect.
He was described as in his 20s or early 30s, and was wearing dark denim type trousers and a dark coloured jacket.
Det Insp Clark said the dramatic chain of events began 10 minutes before the shooting with an altercation at the Barcode nightclub in nearby King Street, during which another woman was punched or kicked in the face.
Police had not been able to establish what sparked that dispute, he said.
The nightclub was sealed off as a crime scene and police are questioning a different man arrested there.
Inspector Glenn Weir said it appeared the shooting was a domestic incident between the gunman and the woman, and not a gang-related or random act.
"It does appear that there was a relationship between the female and the male suspect and certainly we're not looking that it's a random act, certainly not gang-related, it appears as though it's a domestic-related incident."
Det Insp Clark said a police search of the area turned up a handgun and a black coat, both believed to belong to the gunman, at a building site in Bond Street.
"We've located a firearm, a handgun, in the near vicinity and a dark coloured jacket. Whilst we can't confirm it was the same firearm used in the shooting, it's the same calibre," he said.
A mobile phone found on Flinders Street was also being examined.
As heavily-armed police searched for the gunman, police cordoned off a crime scene area between Flinders Lane and Flinders Street, William and Market streets.
City office workers were told to stay put and not panic as police sealed off some offices and evacuated others.
Police interviewed up to 10 witnesses to the shooting inside the foyer of 15 William Street before they were taken by minibus to the Victoria Police Centre for further questioning.
