Foreign farm workers were among those killed in the crash involving two cars and a truck at the intersection of Murray Valley Highway and Labuan Road at Strathmerton about 2.30pm on Thursday.
Police had intercepted a white Mercedes sedan about 1.45pm on Labuan Road and gave the 29-year-old male driver a speeding ticket for going 118km/h in a 100km/h zone.
Fewer than two kilometres away, the Mercedes collided with a Nissan Navara, which spun out and hit a fully loaded B-double milk tanker on the highway.
All five people in the Navara died instantly.
Two other people involved in the crash, the 29-year-old Mercedes driver and his 29-year-old female passenger, were taken to hospital with minor injuries and since released.
The driver, who is from Melbourne's eastern suburbs, is in police custody and being interviewed.
Victoria Police Assistant Commissioner Glenn Weir said he had watched footage of the incident from the truck.
"It's horrific. It is catastrophic, the damage that has been caused as a result of a simple collision that has ended with the loss of five lives," he told reporters on Friday.
Police from the major collision investigation unit along with local officers and other emergency services worked through the night to investigate the crash, Mr Weir said.
The assistant commissioner described the scene as terribly confronting and tragic.
The damage was so severe that police had to call in disaster victim identification experts and a search-and-rescue team to determine how many people had died, Mr Weir said.
Identification issues led to delay in police revising the death toll to five people, after officers initially said on Thursday that three people had been killed.
Among the dead was the Nissan's driver, a woman in her 60s who police believed was local to the area.
The remaining passengers who died were one male and three females, who police believed were Asian and doing farm work for the woman in Australia.
Victoria has not had five people killed in a single crash since 2012, Mr Weir said.
Labuan Road has a give way sign at the intersection with Murray Valley Highway, and police will allege the Mercedes' driver disobeyed it, he said.
"At some stage, I anticipate that charges will be laid and the judicial process will ... go through its normal course," Mr Weir said.
"However, the ripple effect of this one tragic incident will remain for a long time."
Mr Weir pointed to the effect on first responders, the devastation that would be felt in the community of Strathmerton where "everyone knows everyone" and the heartbreak that would crush families overseas.
The truck driver was absolutely devastated, Mr Weir said.
"And so, when all the tape is taken down, the cars are towed away, everything's cleaned up and everyone will move on, it's quite easy for that to be something that happened the other week," he said.
"We can't let it stop at that."
Victoria has been devastated by significant road trauma this year, and more so than in 2022, Mr Weir said.
"The people who won't travel (this long weekend) are the people who are dead," he said.
"It's time for people to wake up."
Police would consult with overseas embassies through the identification and notification process, Mr Weir said.
The investigation is ongoing, and police intend to ask "hard questions" of local authorities following previous road trauma in the area.
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