The lawyer for the truck driver charged over the deaths of four police officers in a freeway crash in Melbourne last week says his client feels distressed and may have an undiagnosed psychiatric disorder.

Key points: Cranbourne man Mohinder Singh, 47, will appear in court again in October

Cranbourne man Mohinder Singh, 47, will appear in court again in October He was kept in hospital for several days, after suffering what police described as a "medical episode"

He was kept in hospital for several days, after suffering what police described as a "medical episode" He was charged early this morning after being released from hospital

Leading Senior Constable Lynette Taylor, Senior Constable Kevin King, Constable Glen Humphris and Constable Josh Prestney were killed when the truck, being driven by Mohinder Singh, crashed into them about 5:30pm on Wednesday, after they had intercepted a Porsche on the Eastern Freeway at Kew.

The four officers had been standing in the emergency lane of the freeway as they prepared to impound the Porsche 911, which was being driven by 41-year-old Melbourne mortgage broker Richard Pusey.

Police allege Mr Pusey was driving at 149 kilometres per hour when they made the decision to pull him over.

Victoria Police Chief Commissioner Graham Ashton said on Thursday that the truck appeared to have moved from the right-most lane of the freeway to the emergency lane shortly before it hit the officers while travelling at 100 kph.

The officers all died at the scene.

Senior Constable Kevin King, Leading Senior Constable Lynette Taylor, Constable Joshua Prestney and Constable Glen Humphris were killed in the crash last Wednesday. ( Supplied: Victoria Police )

Mr Singh, 47, had been in hospital since Wednesday night after suffering what Chief Commissioner Ashton described as a "medical episode … which seemed to involve him blacking out".

He was released from hospital and questioned by police about the crash for the first time on Sunday night, before he was charged with four counts of culpable driving causing death.

Tributes to the four officers were left outside the Boroondara Police Station last week. ( ABC News: Billy Draper )

During a brief hearing in the Melbourne Magistrates' Court this morning, Mr Singh's lawyer Steven Pica said his client felt "deeply saddened and distressed by the tragic consequences" of the crash, and had been suffering from anxiety and panic attacks.

Mr Pica also said his client may have an undiagnosed psychiatric illness.

Mr Singh appeared unshaven and had his head bowed for most of the hearing.

He did not apply for bail and was remanded in custody to appear in court again in October for a committal mention.

Mohinder Singh is facing four counts of culpable driving causing death. ( Facebook )

Police last week conducted a search of Mr Singh's Cranbourne home.

Mr Pusey has been charged with nine offences including reckless conduct endangering life, failing to render assistance and drug possession.

Magistrate Kieran Gilligan said today that the cases of Mr Singh and Mr Pusey would be run separately to one another.

Police urge any remaining witnesses to come forward

At a brief press conference, Assistant Commissioner Libby Murphy said officers had executed two search warrants at Croydon and Frankston linked to Mr Singh's employer Connect Logistics.

Assistant Commissioner Murphy said the investigation of the company was standard procedure for any serious crash involving heavy vehicles.

She also urged anyone who saw the truck in the lead-up to the crash or had dashcam footage to contact Crime Stoppers.

The prime mover had the Queensland registration XV8 5IE and the trailer had the registration 84I QWK.

Connect Logistics said in a statement this afternoon that it was "devastated by the loss" of the four officers.

Melbourne's major buildings were illuminated in blue last week to honour the four police officers killed in the crash. ( ABC News: Gemma Hall )

"Our prayers and deepest condolences go out to the families, Victoria Police and everyone dealing with this horrific loss," the statement said.

"As the issue is still under investigation it is not appropriate for us to comment any further.

"We remain heartbroken by this tragedy."

Assistant Commissioner Murphy said police were also keen for anyone who stopped to help the four officers on Wednesday to come forward as potential witnesses and call Crime Stoppers.

"We are not sure yet whether we have captured everyone who stopped in terms of identifying them as witnesses," she said, adding that those who came to the aid of the officers had the force's "heartfelt thanks".

Assistant Commissioner Libby Murphy said the deaths had been "heartbreaking" for Victoria Police. ( AAP: James Ross )

Assistant Commissioner Murphy, who went through the Victoria Police Academy with Leading Senior Constable Taylor 31 years ago, said people must understand that their behaviour on the road could have fatal consequences for others.

She also thanked the public for their displays of support, including the lighting up of buildings across Melbourne and Australia last week.

"It's actually quite overwhelming for us and our membership, but thank you, thank you, thank you," she said.