Two children who were inside the 4WD that rolled down a paddock have since died after the crash at Parwan, west of Melbourne, shortly before 4pm on Tuesday.
A seven-year-old girl was killed at the scene, while an 11-year-old boy was flown to hospital with critical injuries but died overnight.
Police believe the car was travelling west on Nerowie Road on the city's western fringe when it "inexplicably rolled".
"We don't know why,'' Detective Acting Sergeant Tony Gentile said at the scene on Tuesday night.
Five other children were inside the car, with two primary school-aged boys rushed to Melbourne's Royal Children's Hospital.
One was airlifted with upper and lower body injuries in a stable condition, while the second boy was taken by road with upper body injuries.
Paramedics treated a teenage boy and primary school-aged boy both for upper body injuries at Western Sunshine Hospital.
A female toddler was taken to the same hospital with a minor upper body injury in a stable condition.
The driver, a 34-year-old woman, remains at the Royal Melbourne Hospital in a stable condition.
Police worked through the night to clear the crash scene, with the eight-seater car removed by Wednesday morning.
Possessions from the vehicle were left scattered along the roadside, including a tablet, water bottles and a black shoe.
Marks from the car could be seen where it veered off the road, which has a speed limit of 100km/h.
Det Act Sgt Tony Gentile described the scene as "quite hectic", with several people witnessing the crash.
"There were people everywhere," he told reporters at the scene.
"This is just a really sad set of circumstances."
Specialist ambulance resources remained at the crash scene, near the Nerowie and Parwan-Exford roads intersection, into the evening.
Among them were two helicopter crews, two mobile intensive care ambulance paramedic crews and five advanced life support paramedic crews.
The crash occurred not far from an intersection in Eynesbury where another horror crash occurred in May 2023.
About 45 students were on a bus when a truck crashed into the back of it, leaving eight children with life-threatening injuries.
Two students needed to have arms and hands amputated.
The driver of the truck was cleared by a jury of all charges over the crash in the County Court in 2025.
17°C