Free-diver hit by boat off Rottnest Island

A free-diver hit by a boat off Rottnest Island is in a serious but stable condition in Royal Perth Hospital.

A man believed to be in his 30s was hit by the vessel near Thompson Bay and police were called about 12.45pm on Saturday.

He was taken aboard the boat to land, where he was picked up by the RAC rescue helicopter and flown to hospital, a police spokeswoman told AAP.

It is believed the man may have back injuries, she said.

Free-diving is a form of diving that relies on a person holding their breath, rather than using scuba gear.