Melbourne crash: Four Victoria Police officers killed by truck on Eastern Freeway

Four police officers have been killed on a Melbourne freeway when a truck ploughed into their vehicles after they pulled over a Porsche 911 driver on Wednesday night. 

The Porsche driver had been pulled over by two Victoria Police officers at roughly 5:40pm (local time) for travelling at an excessive speed while entering the Eastern Freeway near Kew.

The two officers then called for back-up, Chief Commissioner Graham Ashton told reporters at the scene.

Shortly after the arrival of a second police car, it's understood that a semi-truck veered off the freeway and ploughed into the two vehicles and the Porsche.

Two senior constables, male and female, and two other constables were killed at the scene.

As reported by 7 News, one constable had only recently joined the force.

The truck driver has been taken to hospital after suffering a "medical episode", local media report, and remains under police guard. Blood samples will be taken to determine whether the driver was under the influence of drugs or alcohol.

The driver of the Porsche is believed to be uninjured and fled the scene of the crash on foot.

Commissioner Ashton told reporters that he "strongly suggests" the man hands himself in to the nearest police station.

"Tonight is a very tragic night and we will do our best to provide answers on it as soon as we can," he said.

Homicide Squad Detectives and the Major Crash Investigation Unit attended the crash site. 

In a statement to 7 News, the Police Association Victoria said it is "shocked and immensely saddened" by the officers' deaths and immediate and ongoing support is being provided to their families and colleagues.

"This is a brutally sad reminder of the danger police face in the course of their service, every minute of every shift... We mourn their loss, we grieve with their families and colleagues and we will never forget them."

As of early on Thursday morning, the driver of the Porsche has not been located.