Four children, aged eight to 15, and a 40-year-old spectator have been injured, with a 4WD car ploughing into the crowd at a mud-racing event in Victoria.

The crash happened just after 4pm (AEDT) on Sunday during The Mud Run, being held at Dalyston Recreation Reserve near Wonthaggi in southeast Victoria.

The vehicles were racing around a mud track when one lost control and veered into the crowd sitting behind temporary fencing, a police spokeswoman said.

An Ambulance Victoria spokesman said four children were treated at the scene by paramedics and then taken to hospitals.

A 40-year-old-man with multiple fractures was taken by road to the Alfred hospital in Melbourne, while two girls with broken legs, one aged 10 and the other 15, were airlifted to the Royal Children's Hospital.

An eight-year-old boy with bruising to the knees was taken to Wonthaggi Hospital and a nine-year-old girl with hip injuries to Dandenong Hospital.

All the children were in a serious but stable condition.

The driver of the car was not injured.

The event was organised by the Australian 4WD Mud Racing Association.