A police officer who shot dead a Melbourne driver during a routine traffic stop believed his life was in immediate threat.
Senior police have defended the highway patrol officer's actions, saying he felt he had no choice but to open fire.
The senior constable, who was working alone, fired three shots at close range and killed the 44-year-old man onSunday night.
The driver had been pulled over in the city's inner south because the car he was driving had suspected stolen number plates.
Police say the officer was trying to arrest the driver in Windsor about 10.40pm (AEST) when the man allegedly pulled out a flick knife.
Assistant Commissioner Luke Cornelius says the officer then fired his gun to defend himself. "Our officer believed that his life was in immediate threat," he said.
"It's a terrible decision to have to make." He said the officer was experienced and respected by his colleagues. "He certainly knew what he was doing."
A woman who was in the car when the man was shot on Union Street fled and police want to track her down. Nearby resident Helen Jones said she heard the gunfire while walking her dogs.
"It was just three shots. Bang, bang bang. Like that," she said. Police Association secretary Greg Davies said any police shooting is a tragedy for all involved. "Regardless of the justification, that man's got to live with that for the rest of his life," he said.
But the decision by police to publicly back the officer drew criticism from justice advocates.
Michelle McDonnell, spokeswoman for Federation of Community Legal Centres, said such comments contravene the force's own guidelines to not pre-judge the outcome of an investigation.
"The police officer may have acted entirely appropriately, but it's up to the coroner to make that outcome further down the track," she said.
Mr Cornelius denied he had prejudged the outcome, saying the conclusions police draw will be tested at the inquest and he had a duty to his officers.
"Bottom line is, my members need to know that they're supported." It has been three years since the last fatal Victoria Police shooting.
Homicide detectives and officers from the internal professional standards command are heading up the latest investigation.
The officer's unmarked patrol car's dashboard camera may have captured the incident and there may be CCTV footage from nearby businesses.
Police would not comment on any specifics of the dead man's identity other than to confirm he lived in St Albans.
His family has been notified.
