The four Victoria Police officers killed in Wednesday's truck crash have been remembered as a veteran of the force with a "great sense of adventure", a "terrific bloke" with a strong sense of humour, a new recruit with a "great sense of empathy" and a "much loved" recent graduate.

Key points: Constable Glen Humphris and Constable Josh Prestney were on extended training placements

Constable Glen Humphris and Constable Josh Prestney were on extended training placements Leading Senior Constable Lynette Taylor and Senior Constable Kevin King were remembered as passionate members of the force

Leading Senior Constable Lynette Taylor and Senior Constable Kevin King were remembered as passionate members of the force Flags have been flown at half-mast in honour of the fallen officers

Leading Senior Constable Lynette Taylor, Constable Glen Humphris, Senior Constable Kevin King and Constable Josh Prestney died after being hit by a truck late Wednesday afternoon.

It is the biggest loss of officer lives in a single event in Victoria Police's history.

Leading Senior Constable Taylor and trainee officer Constable Humphris pulled over a Porsche that was allegedly driving at 140 kilometres per hour along the freeway.

Senior Constable King and trainee officer Constable Prestney, who were stationed at Nunawading Highway Patrol, were called to help impound the sports car after the driver allegedly returned a positive drug test.

Shortly after they arrived, a refrigerated semi-trailer drove into the emergency lane, killing all four officers.

The truck driver was taken to hospital under police guard after having a medical episode and the driver of the Porsche, mortgage broker Richard Pusey, has been questioned by police after allegedly fleeing the scene.

The Australian flag is being flown at half mast at Wedderburn Police Station to honour the officers who died. ( ABC News: Tim Lee )

"Their names will forever be remembered, their service honoured, and their memories celebrated," Chief Commissioner Graham Ashton said.

"These officers were undertaking their everyday duties in helping to keep the community safe when they were killed.

"They were our colleagues, our friends, our squad mates, our family.

"Each from different backgrounds, each with different life experiences, each with a common goal to keep the community safe."

Leading Senior Constable Lynette Taylor had a 31-year career with Victoria Police. ( Supplied: Victoria Police )

Leading Senior Constable Taylor, 60, served with Victoria Police for 31 years after joining the force in 1989.

She is survived by her husband, former Victoria Police officer Stuart Schultz, and their two sons, Nathan and Alexander.

"Having travelled the world, Lynette is remembered for having a great sense of adventure and has instilled this spirit in her children," a Victoria Police obituary read.

Leading Senior Constable Taylor once spent a year sailing around the South Pacific Ocean on a yacht.

She and her husband were building their "dream retirement home" on Victoria's south-east coast, the obituary said.

She was recognised by the Chief Commissioner for her work at the Traffic Camera Office and received the National Medal first clasp — 25 years, National Police Service medal and the Victoria Police Service medal fourth clasp — 30 years.

Senior Constable Kevin King was remembered as an officer who "always made good judgement calls and decisions". ( Supplied: Victoria Police )

Senior Constable Kevin King, 50, was remembered as a "terrific bloke" who was passionate about road policing.

He joined Victoria Police six years ago and had been with the Nunawading Highway Patrol since 2018, where the Officer in Charge described him as "an older head with a lot of life experience who always made good judgement calls and decisions".

He is survived by his wife Sharron Mackenzie and their three children William, James and Henry.

"[He] was well liked, well respected and well regarded by his colleagues. He had a great sense of humour, loved coming to work, and he loved what he did," a Victoria Police obituary read.

Constable Glen Humphris was remembered as a new recruit to the force who held "genuine empathy for everyone". ( Supplied: Victoria Police )

Constable Glen Humphris was remembered in a police obituary as a new recruit with a "great sense of community and genuine empathy for everyone — victim and offender alike".

Before joining the force, Constable Humphris had worked as a carpenter, personal trainer and sports scientist in New South Wales.

He then moved to Melbourne with his partner, Todd, and began training at the Police Academy to become an officer — scoring near the top of his class in his exams.

"Glen is remembered as being very level-headed, caring and supportive with a great sense of humour and had the ability to make light of any situation to calm people," the obituary said.

Twenty-eight-year-old Victoria Police Constable Joshua Prestney had recently joined the force. ( Supplied: Victoria Police )

Constable Joshua Prestney, 28, was mourned as a "much loved and respected member of his squad".

His brother, First Constable Alexander Prestney, presented him with his badge when he graduated in December last year, before he was stationed to Boroondara.

Outside his policing, he competed in triathlons and enjoyed keeping fit.

Messages of thanks were left at Boroondara Police Station. ( ABC News: Simon Tucci )

Force honoured after 'terrible tragedy'

Police chaplains and Chief Commissioner Ashton have been visiting the officers' grieving families.

Victoria Police Senior Chaplain, Jim Jung, told ABC Statewide Drive that he had the difficult job of being with the family of Senior Constable Lynette Taylor when they received the "ghastly news" about the accident.

"Unfortunately we needed to knock on their door last night and begin the process of talking to them about what had taken place. And allowing the initial shock and grief to settle in," he said.

"We will then continue now to work with them to ensure we bring the greatest honour to her memory and support them through their time of great grief."

Recruits, instructors and staff at the Victoria Police Academy observed a minute's silence for their fallen colleagues on Thursday morning.

A minute's silence was observed at the police academy. ( Facebook: Victoria Police )

"Our members didn't come to work that day, no doubt, expecting not to go home, their families wouldn't have waved them goodbye expecting them not to return," Police Association Victoria secretary Wayne Gatt said.

"It is deeply, deeply troubling for us to watch those families grieving."

Due to the coronavirus pandemic, a full honours funeral will not go ahead in the near future.

The officers have been hailed as heroes by Premier Daniel Andrews and Prime Minister Scott Morrison extended his "deepest sympathies and condolences" to their families and colleagues.

"On behalf of every single Victorian, I send the best wishes, the thoughts and prayers of all of us to the families of the four members of Victoria Police who died keeping the rest of us safe. This is a terrible tragedy," Mr Andrews said on Thursday morning.

Members of the public have left flowers and notes at Boroondara Police Station. ( ABC News: Simon Tucci )

Mr Morrison said it was a reminder of the dangers officers faced every day.

"You stand between us and that danger every single day and we are deeply grateful for your service, deeply grateful for your sacrifice," he said.

"And to those families who are knowing nothing other than terrible grief today, we stand with you as much as we possibly can."

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 1 minute 42 seconds 1 m 42 s Police Association secretary Wayne Gatt said it was an "unimaginable time".

Mourners left flowers and messages of thanks at the Boroondara Police Station in Kew while Chief Commissioner Ashton urged people to instead leave tributes online during the COVID-19 pandemic.

Former Victorian deputy premier Pat McNamara told ABC Statewide Drive his son was a friend of one of the trainees, who had recently graduated.

"These incidents really shock people to the core," he said.

"To think a life cut short, well, four lives cut short, there are families, there are friends, all those officers have got people who are grieving, who know them personally and obviously they have a wide circle of friends too. Everybody is shocked by the incident. "

Landmarks across Victoria were lit up in blue on Thursday night in honour of police officers.