Australian officials are hunting a shark that killed a teenager off Australia's southwest coast in the nation's second deadly attack in as many weeks.

Jay Muscat, 17, was spearfishing with a friend on Monday off Cheynes Beach, on the south coast of Western Australia (WA), when he was attacked.

The shark is believed to be a great white measuring between 4m and 5m (13ft and 16 ft) long, said Carlo Vittiglia, spokesman for the state fisheries department.

There is a chance the animal was injured, Vittiglia said, as Mr Muscat's friend is thought to have fired a spear at the shark during the attack.

His friend Matt Pullella wrote on Facebook that "the shark hit me first then attacked Jay", The West Australian reported.

"The shark turned and came for me, I pushed the speargun down its throat and fired the gun!" he wrote. "This is something no one should ever have to see."

WA's Department of Fisheries said Cheynes Beach would remained closed while equipment was deployed from boats to try and catch the shark.

If the shark is caught, it will be destroyed.

"One of them (boats) will be setting (drum) lines, the other will be doing patrols in the nearby regions," Department of Fisheries spokesman Rick Fletcher told the Australian Broadcasting Corporation.

The attack comes two weeks after an 18-year-old man was killed by a shark while spearfishing on the Great Barrier Reef off Australia's east coast.

Sharks are common off Australia's beaches, but fatal attacks are rare.

The country has averaged fewer than two deadly attacks per year in recent decades, but experts say attacks are becoming more common as water sports increase in popularity.