"He has attempted to sit on its back and the croc has taken offense to that and has spun around and bit him on the right leg," Broome Police Sgt. Roger Haynes said.
The saltwater crocodile then inexplicably let the man go, and he climbed back over the fence to safety, police said.
The man, who was a tourist from eastern Australia and whose name was not released, suffered some "very nasty lacerations" and was taken to a hospital, Haynes said.
"Saltwater crocodiles ... once they get hold of you, are not renowned for letting you go," Haynes said. "He's lucky to have escaped with his life."
Saltwater crocodiles are the world's largest reptile and can grow up to 23 feet (7 meters). They have become increasingly common in Australia's tropical north since hunting that almost extinguished the species was banned in 1971.