Police are trying to find the owner of a motorbike who assaulted a 39-year-old man after he stole and crashed his Harley-Davidson.

Police say the man stole the bike at Greenwell Point near Nowra on the New South Wales south coast yesterday afternoon and was taking it for a ride around the township when he lost control of it.

Police say the accident caused the bike to cartwheel down the street, while the rider ended up in a garden bed with a stake embedded through his right arm and out of his back.

Police say the owner of the bike found the man a short time later and took his bike back, but not before punching him in the face and pulling the stake out of his torso.

Ambulance paramedics arrived shortly after to treat him.

He was then airlifted to Sydney's St George Hospital with a broken nose and internal injuries.