A YOUNG girl has been killed in a quad bike accident on Thistle Island, near Port Lincoln, on South Australia's Eyre Peninsula.

The girl, 8, received critical head injuries in the accident at 5.50pm.

She died as a retrieval helicopter was flying to the accident scene.

Major Crash investigators are heading to the location. A report will be prepared for the State Coroner.

Meanwhile, an 11-year-old boy has been rushed to hospital with life-threatening injuries after falling off his motorbike in a paddock at Gulnare, 60km north of Clare.

The incident happened 300m from a farmhouse on Old Coach Rd, about 1pm.

The boy was flown to the Royal Adelaide Hospital in a critical condition with serious head and chest injuries and was later transferred to the Women's and Children's Hospital.