Police in Melbourne are searching for a Porsche driver who fled the scene of a traffic accident in which four officers were killed.

Key points: The officers had pulled over a Porsche for speeding

The officers had pulled over a Porsche for speeding A truck drove into the officers and the Porsche, which became stuck under the truck

A truck drove into the officers and the Porsche, which became stuck under the truck The driver of the Porsche, who is not believed to have been injured, fled on foot

Victoria Police Chief Commissioner Graham Ashton said it was the largest single loss of lives in one incident in the history of Victoria Police.

"Our thoughts and prayers are with the families of our officers tonight," he said.

He said the tragic incident highlighted the dangers of everyday police work.

"All the officers down here … I can tell you there's an eerie quiet down here … everyone down there understands what a tragedy this is for the families of those officers, but it's a tragedy for the whole force," he said.

Chief Commissioner Ashton said police were still working to establish the identity of the Porsche driver and urged the man to give himself up to police.

Police Association Victoria secretary Wayne Gatt was visibly shaken and emotional when he spoke to the media, saying he was "floored" by the deaths of the "four dedicated members".

"This is the bread and butter of policing. This isn't stuff that you … expect to be killed doing," he said.

"Four of our community paid the ultimate sacrifice for protecting all of us.

"I can't express to you how deeply felt that will be in the policing family tonight and in coming weeks."

The two senior constables and two constables were killed in the crash in the citybound lanes of the freeway near the Chandler Highway at Kew around 5:40pm, after they had pulled over a black Porsche for speeding.

Chief Commissioner Ashton said two of the officers had pulled the Porsche over in the emergency stopping lane when they decided to impound the car and called for two more officers to assist them.

Two other officers arrived on the scene and all were out of their cars when a large refrigerated truck then drove into the officers and the Porsche.

The driver of the Porsche, who is not believed to be injured, fled the scene on foot and is still at large.

The driver of the truck will be tested for drugs and alcohol. ( AAP: David Crosling )

Truck driver under police guard in hospital

Chief Commissioner Ashton said the truck driver, who is believed to work for a poultry business in Melbourne, had some sort of medical episode, but it is not clear if that happened before or after the crash.

Chief Commissioner Ashton said it was not clear where the officers were when they were hit by the truck, which crushed the Porsche.

The truck driver is under police guard in hospital where he will be tested for drugs and alcohol.

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"At the moment he's unable to assist us with the initial inquiries at the scene," Chief Commissioner Ashton said.

"We intend for him to be there [in hospital] under police guard for some time. When it's possible for him to be interviewed he will be interviewed."

One of the officers had only recently joined the force, Chief Commissioner Ashton said, while the others were more experienced.

Officers hailed as 'heroes' by Premier

One of the senior constables was female, while the other three officers were male, he said.

The Homicide Squad will lead the investigation, assisted by the Major Collision Investigation Unit and Professional Standards Command.

The coroner was also at the scene.

"In these type of events we have to do an investigation as to criminality … those officers concerned have been the victims of a crime, so the Homicide Squad will have primacy of that," Chief Commissioner Ashton said.

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Victorian Premier Daniel Andrews hailed the four officers as "heroes", in a statement acknowledging the heartbreak being suffered by the officers' families.

"We grieve alongside them — just as we grieve with every member of Victoria Police and every member of our emergency services family," he said.

"In the coming days, formal investigations will tell us why — how — this could possibly have happened.

"But one thing is already clear: Though we may not yet know their names — we will always call them heroes."

Prime Minister Scott Morrison also took to social media to send his "deepest sympathies" to the family and colleagues of the officers.

Victorian Opposition Leader Michael O'Brien offered his condolences to those close to the officers killed in the line of duty.

"This unimaginable tragedy is a stark reminder of the grave risks the members of Victoria Police face every day," he said.

The freeway had been closed in both directions between the Chandler Highway and Bulleen Road.

Police Minister Lisa Neville said the "shocking" news of the deaths would be felt across the force.

"We rely on police … they do an incredible job every single day," she said.

"So when someone is injured or harmed in the line of duty it hurts every one of us."