A seven-year-old has died after a quad bike accident on an island off South Australia's Eyre Peninsula.

Police say the crash happened about 5:45pm on Sunday on Thistle Island, about 40 kilometres south-east of Port Lincoln.

The girl sustained serious head injuries and died at the scene.

Major crash investigators are travelling to the island from Adelaide.

Her death takes the state's holiday road toll to five.

In an earlier incident, an 11-year-old boy suffered critical injuries in a motorbike accident on a property in South Australia's mid north.

He was airlifted to the Adelaide Womens and Children's Hospital where his condition is now described as stable.

Police say he was found unconscious in a paddock near Crystal Brook on Sunday afternoon.

at the Shakespeare Hills Forest Reserve in the state's north-west.