Update 8pm Thursday
A Warrnambool police sergeant says the child airlifted to hospital was a three-year-old girl.
Sergeant Bec Miles said a 30-year-old male driver was travelling west on the Princes Highway when he collided with a second vehicle travelling east towards Warrnambool.
"The man has collided head on with a mother and her three children," she said.
"A three-year-old girl was airlifted to Melbourne with head injuries. The mother, her baby and four-year-old child have been treated at Warrnambool Base Hospital for shock and minor injuries."
Sergeant Miles said witnesses told police that the male driver appeared to be driving erratically in the lead up to the collision.
She said it was believed the man might have been suffering from a medical condition.
Emergency crews are still working to clear the scene.
Road diversions are in still place between Southern Cross and Lake View roads.
Update 7.30pm Thursday
Warrnambool SES has worked with paramedics to free a trapped male from one of the vehicles.
Unit controller Giorgio Palmeri said the vehicle, which came to rest on its roof after rolling, had to be stabilised using shoring equipment to safely remove the victim from the car.
“SES Rescue operators then used hydraulic rescue cutters and cut the doors and b-pillar away to access and extricate the man from the car,” he said.
12 members of the unit worked for about 20 minutes to free the man aged in his 30s. He was taken to Warrnambool Base Hospital in a stable condition.
Warrnambool and Port Fairy SES units have remained on the scene to provide lighting while police conduct their investigations and the scene is cleared.
Koroit CFA captain Leigh Mugavin said pumpers from Koroit and Warrnambool attended the scene as well as a tanker from Kirkstall.
He said there was a small amount of fuel leaking from one of the vehicles.
“We cleaned that up and made sure that the scene was safe,” he said.
Earlier 6pm Thursday
A primary school aged girl will be airlifted to hospital and a further three children and two adults are being assessed by emergency services after a collision on the Princes Highway near Tower Hill.
A spokeswoman for Ambulance Victoria said one vehicle rolled at the intersection of the Princes Highway and Lake View Road at 5.01pm.
She said a man in his 30s was initially trapped in a vehicle.
He has since been freed and is being taken by road ambulance to Warrnambool Base Hospital in a stable condition.
A woman in her 30s has injuries to her hand.
A preschool aged child is being assessed at the scene with ankle injuries and is in a stable condition.
A preschool aged child is being assessed at the scene with no obvious injuries.
A preschool aged boy is being assessed at the scene with injuries to his head.
A primary school aged girl is being assessed at the scene with head injuries and will be airlifted to a Melbourne hospital.