A 51-year-old man who disappeared while swimming with his son near an Australian beach today may have been killed by a shark, police said.

The man was at a beach south of Perth, capital of Western Australia state, when he suddenly disappeared from view in the early morning, Sergeant Peter Whitney told Reuters.

He was with his 24-year-old son at the time, who had earlier seen a shark in the area, police said. Another witness had also seen the man struggling in the water.

"The son saw a shark in the area and lost sight of his father," Whitney said by telephone. "It is presumed at this stage to be a shark attack. The information from witnesses suggests this is a confirmed shark attack."

The attack took place at Port Kennedy, on the Indian Ocean coast to the south of Perth. Police searched the area with boats, police on horseback and vehicles but as of late afternoon had found no trace of the man.

"At this stage they have not located the missing person," Whitney said.

Australian Broadcasting Corporation named the man as Brian Guest, a banker, and said he had been snorkelling for crabs when the attack occurred and was just six metres from the shore at the time.

Sharks are a regular threat around Australian beaches and attacks on humans, although uncommon, periodically occur.