The 41-year-old man, who is known to police, is now being interviewed by homicide detectives after fleeing the Kew crash last night.

"Police have located the driver of a Porsche believed to be involved in the fatal collision involving the death of four police members in Kew yesterday," a police statement said.

The investigation into the exact cause of the collision remains ongoing.

The man was behind the wheel of a Porsche that was pulled over by officers on the Eastern Freeway on Wednesday about 4.50pm and he had tested positive to drugs.

More officers were called to help impound the car and while all four police were standing by the side of the road about 5,40pm, a truck ploughed into them.

The Porsche driver ran from the scene and it's alleged he took photos of the crash and shared them on social media before going into hiding overnight.

Coroner Darren Bracken will run the investigation and has told Victoria Police to compile a coronial brief of evidence.

Flags are flying at half-mast across Victoria on Thursday in honour of the female senior constable, male senior constable and two male constables, one of whom only started work on Tuesday.

Two of the officers had pulled over a black Porsche 911, after it was clocked at 140km/h on the freeway.

The truck driver had a medical episode after the crash, blacked out and is in hospital under police guard.

Police raided the truck driver's Cranbourne home on Wednesday night and are yet to reveal what they found.

Premier Daniel Andrews asked all Victorians to pause and honour Victoria Police officers for the work they do.

More than $37,000 has been raised towards the $2 million target for the families of the deceased police officers via a GoFundMe page.

The semitrailer was removed from the freeway on Thursday morning, but the freeway remains closed between Bulleen Road and the Chandler Highway.

At 10.30am: The 41-year-old Porsche driver apparently took photos at the crash site and shared them on social media, Victoria Police Chief Commissioner Graham Ashton told reporters.

"As a chief Commissioner to have four officers killed last night - someone who has clearly been driving erratically on the freeway with an extensive criminal history, it disgusts me," Mr Ashton said.

"And then to leave the scene is a very, very low act in my view."

The man has since made contact with police and will hand himself in.

At 9.30am: A truck was travelling at 100km/h when it hit and killed four police officers dealing with the driver of a Porsche, who had tested positive to a roadside drug test, on a Melbourne freeway.

Flags will fly at half-mast across Victoria on Thursday in honour of the officers who died when the truck ploughed into them as they stood on the Eastern Freeway at Kew about 4.50pm on Wednesday.

Further details about the female senior constable, male senior constable and two male constables, one of whom only recently joined the force, are expected to be released later on Thursday.

Two of the officers pulled over the black Porsche 911 after they clocked it speeding at 140km/h on the freeway.

The driver tested positive to drugs, back up was called and two more officers arrived.

All four were mowed down by the refrigerated truck but the Porsche driver, who's known to police, ran from the scene uninjured.

"It is an unprecedented event for us to lose so many officers in one event. Officers just doing their work, just doing their job," Victoria Police Chief Commissioner Graham Ashton said on Thursday morning.

The 41-year-old Porsch drive apparently took photos at the crash site and shared them on social media, Mr Ashton said.

The man, who has an extensive criminal history, has since made contact with police and will hand himself in.

The truck driver had a medical episode after the crash, blacked out and is in hospital under police guard.

Police raided the truck driver's Cranbourne home last night and are yet to reveal what they found.

Earlier: Four police officers hit and killed on a Melbourne freeway have became victims of crime while simply doing their jobs.

The constables died when the truck ploughed into them as they stood on the Eastern Freeway near the Chandler Highway at Kew about 5.40pm on Wednesday.

A female senior constable, a male senior constable and two male constables, one of whom had only recently joined the force, are among the deceased.

The driver of a Porsche who was pulled over and fled after the officers were killed, has been urged to contact police investigating the incident.

Speaking at the scene on Wednesday night, Victoria Police Chief Commissioner Graham Ashton said the incident was very tragic.

"We've had four tragic deaths here tonight, and the officers, those officers concerned, have been the victim of a crime," he said.

Two of the officers had intercepted the Porsche, which had been speeding on the freeway, and called for back-up when they decided to impound the vehicle, as another two officers arrived.

Soon after, the refrigerated truck ploughed into the officers.

The Porsche driver, who is believed to be uninjured, 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.

The truck driver is believed to have had a "medical episode" and was taken to hospital under police guard.

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

The truck driver be interviewed and undergo testing for the presence of alcohol or drugs in his system.

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

Police Association Victoria secretary Wayne Gatt said the deaths had sent shockwaves through the policing community.

"The police force is waking up feeling numb, the members, the men and women of Victoria Police challenged deeply by this," he said this morning.

"It is every police officer's worst nightmare, going to the scene of something where an officer has lost their lives and here we have four. I don't know how you even start dealing with that.

"Pulling over a car is routine work for officers who do it up to 30 times in a shift, he said.

Trevor, a cousin of one of the deceased officers who drives a similar truck to the one involved in the tragedy, told 3AW the death of his married cousin hadn't sunk in.

"It hasn't hit home yet, but the job they do, I don't think is respected enough," the man said on Thursday.

"I just had a thing that something wasn't right ... unfortunately it come (sic) to fruition."

Premier Daniel Andrews sent a message of support to the families of the four officers late on Wednesday night.

"Our hearts are breaking with them," he said.

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

The freeway remains closed between Bulleen Road and the Chandler Highway, and drivers are being told to find alternative routes.

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