A woman in her 20s was airlifted to Perth last night after being run down by a four-wheel-drive while riding a bike in Geographe.

The woman and a man, also in his 20s, were cycling east along Marine Terrace about 11.15pm when the car struck them from behind, then drove on without stopping.

The pair were treated at the scene by St John Ambulance and taken to Busselton District Hospital.

The woman’s condition deteriorated, causing medical staff to make the airlift call.

As well as the serious head injury, she also suffered a leg injury.

The man’s injuries were minor.

The woman arrived at Royal Perth Hospital by helicopter at 7.15am and is now in a serious but stable condition, but not in in the intensive care unit.

Police media said they are still investigating the incident and were desperate to locate the vehicle.

They said they had no information on the context behind why the pair were cycling at that time of night.

Police describe the vehicle as a large, dark-coloured four-wheel-drive with possible windscreen damage.

Anyone with information is urged to call Crime Stoppers on 1800 333 000 or lodge a report at crimestopperswa.com.au.