The truck slammed into two police cars and the Porsche before coming to a stop.(Representational)
Four Australian police officers were killed in the line of duty when a speeding Indian-origin driver crashed his truck into them here after he apparently suffered a "medical episode," police said.
The truck driver Mohinder Singh Bajwa from Cranbourne suburb, on Wednesday veered into the emergency lane from one of the freeway lanes and travelled a short distance at around 100 km per hour where the four police officers were standing, said the police chief for the state of Victoria.
Victorian Chief Commissioner Graham Ashton said two of the officers had pulled a speeding Porsche car over in the emergency stopping lane when they decided to impound the vehicle and had called for two more officers to assist them.
The four police officers were out of their cars when the truck drove into the officers and the Porsche.
The truck slammed into the back of one the two police cars and hit the other police car and the Porsche before coming to a stop.
The Porsche was driven by a 41-year-old driver Richard Pusey who later fled the scene on foot and has been accused of taking pictures of the crash and posting them on social media.
Pusey was currently in police custody after he was arrested on Thursday.
Flags are flying at half-mast across the state in honour of one female officer, and three male officers, one of whom only started work on Tuesday.
''''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," Ashton said, adding "And then to leave the scene is a very, very low act in my view."
The truck driver was in the police guard in a local hospital while police raided his residence and were yet to reveal what they had found.
Ashton said that police have yet to interview Bajwa who was not known to the police and did not have an extensive criminal history.
Bajwa suffered a "medical episode", "which seemed to involve him blacking out", Ashton said adding 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.
Leading Senior Constable Lynette Taylor, Constable Glen Humphris, Senior Constable Kevin King and Constable Josh Prestney were identified as four officers who died.
Ashton said that the deaths of the four officers was the biggest loss of life in a single incident in Victorian Police history.
Political leaders have expressed their condolences to the fallen officers.
Prime Minister Scott Morrison tweeted, "Terrible, heartbreaking news that four police officers have died while on duty in Melbourne tonight. My deepest condolences to their families, fellow officers and friends in this terrible time. "