A diver who had his video camera handy has caught dramatic footage of two great white sharks attacking each other in the waters off the south Australia coast.

Adam Malski was in a boat in the waters off the Neptune Islands hoping to observe great whites when an 8-foot shark approached, attracted by some floating bait placed in the water.

Moments later, a second shark almost twice as large surfaced and sank its teeth into the first animal's head.

Malski recognized the larger shark, well known to locals and dubbed Gilbert, because he had seen it earlier in the day during a dive to a depth of 65 feet inside a shark cage.

He described the shark's reaction to his presence as "inquisitive" but "not aggressive."

Later on the boat, as he watched the first shark approach the bait, he asked the boat's captain what might occur if a smaller shark was in the way of a hungry larger one.

"The big shark would take the smaller one down," the captain replied.

That was proved seconds later, said Malski, who is originally from London but now lives in Sydney.

"Literally six seconds later, that amazing scene was witnessed by me, the skipper and the dive master," he said.

"They had never seen anything like it before. I believe that it is extremely rare footage as great whites are elusive and still largely unknown creatures."

While Gilbert has since been seen in the waters around the island, no one has caught sight of the smaller one since the filmed attack.

Great white sharks, which can grow to 20 feet long and are the larges predatory fish on Earth, are though to number less than 10,000 around the world and generally do not fight over food, experts say.

The are found found in coastal surface waters of all the world's major oceans. They reach maturity at about 15 years of age and can live as long as 70 years.