A Victoria Police media liaison spokeswoman said the accident happened about 7.30am when the vehicle collided with a tree on a slight bend along the Casterton-Penola Road, near the intersection with Shanks Road, about 30 kilometres west of Casterton.

The family is believed to be originally from The Congo in Africa.

The mother and her baby girl died instantly. It is understood the woman was thrown from the four-wheel-drive.

The driver, a man aged in his 30s or 40s, was trapped for about two hours before being taken to Mount Gambier hospital in a critical condition.

Ambulance officers are believed to have resuscitated the man, but he died yesterday afternoon in hospital.

Two children, a boy and girl aged five and seven years old, were transported by road to a nearby hospital before being flown to the Royal Children’s Hospital in Melbourne. 

The boy was in a critical condition suffering abdominal, head and pelvic injuries while the girl had minor injuries. Both were listed in a stable condition last night. 

It is believed fatigue may be a factor as the family was travelling from Melbourne back to South Australia.

The four-wheel-drive had South Australian registration plates and received severe front-end damage in the crash. 

Victoria Police major collision investigation unit officers visited the crash scene to try to determine the cause of the accident.

Five ambulances were called to the scene. The Country Fire Authority Casterton rescue unit had to cut people from the wreck. 

Two CFA tankers and State Emergency Service volunteers also provided support. 

Any witnesses are urged to contact Crime Stoppers on 1800 333 000.