The four officers were killed when the truck ploughed into them as they stood on the freeway near the Chandler Highway at Kew about 5.40pm on Wednesday.

The group included a female senior constable, a male senior constable and two male constables, one of whom had only recently joined the force.

"It's a very tragic night for Victoria Police," Chief Commissioner Graham Ashton said at the scene on Wednesday night.

Mr Ashton said it was the largest number of deaths of police in a single incident in the state.

He said two officers intercepted the Porsche, which had been speeding on the freeway, and called for backup when they decided to impound the vehicle, with two other members soon arriving.

Shortly after, the refrigerated truck ploughed into the officers.

The Porsche driver, who is believed to be uninjured, then fled the scene on foot and is still at large.

"I'd strongly suggest he gets himself down to the nearest police station to present himself to assist with these inquiries," the chief commissioner said.

He said police were investigating his identity.

The truck driver experienced a "medical episode" and was taken to hospital under police guard for tests.

But Mr Ashton said it was unclear whether he experienced the episode before or after the accident or exactly what unfolded.

"We don't know at this stage what has caused him to drive that truck into our police members," he said.

The homicide squad is investigating the incident and the coroner was at the scene.

Police Association Victoria secretary Wayne Gatt said the deaths would "shock police in stations across Victoria".

"These police officers were doing something that we do 10 times, 20 times, 30 times a shift - intercepting a motor vehicle on the side of the road, this is the bread and butter of policing," he told reporters.

"This isn't stuff that you go to do and expect to be killed doing. But each and every time a police officer does that for the next little while, they'll have this in the back of their minds, there's no doubt."

Images from the site show a damaged police highway patrol car and an SUV that had been struck near a large, white semi-trailer.

The black Porsche is crumpled beneath the truck and has been covered in a blue tarp.

The freeway's lanes are closed in both directions between Bulleen Road and the Chandler Highway, and drivers have been told to find an alternative route.

Police Minister Lisa Neville said the tragedy hurts the whole community, encouraging people to extend their support to the force at this time.

"Now is the time for people to really thank them," she said.

Australian Associated Press