Police said two boats from the squadron reported sighting a shark about 50 metres offshore from the West Beach Caravan Park, near the spot where Nick Peterson was attacked yesterday afternoon.

So far, no remains have been retrieved of the young paver who was said to have taken the day off work to go to the beach, according to a report by the Australian newspaper quoting a friend, Louise French.

The teenager, from South Australia’s Yorke Peninsula but living in Adelaide, was being towed on a surfboard behind a boat about 300 metres from the shoreline of the popular beach when a shark grabbed one of his arms and dragged him under.

Three of Mr Peterson’s friends watched in horror from the boat as the shark tore their mate in half, before a second great white took his remains.

SA Sea Rescue Squadron spokesman Frasier Bell said one of the sharks was about 5 metres long and the second was roughly 4.5 metres in length.

Mr Bell said Mr Peterson’s three 16-year-old friends who witnessed the attack were deeply shocked by his death which they had been unable to stop.

Temperatures are forecast to hit the mid-30s in coming days, but beachgoers have been warned to keep to patrolled areas of the Adelaide coast.

Surf lifesaving clubs are also expected to put on extra patrols over the next few days while fears remain of the sharks returning to the area.

It is the second fatal shark attack in the country in a week, after 38-year-old Mark Thompson was killed at Opal Reef, 75 kilometres from Cairns, on December 11.

Mr Thompson suffered massive leg wounds after being bitten and died of a cardiac arrest before medical treatment arrived.

Yesterday, South Australia’s Surf Life Saving spokesman Shane Daw said there had been no decision made about whether to destroy the sharks responsible for Mr Peterson’s death.

South Australia, with its rich feeding grounds in waters such as the Spencer Gulf, has become a hotspot for fatal shark attacks, according to shark expert Rodney Fox.

Ten deaths have been recorded in the state since 1975.

John West, keeper of the national Shark Attack File and operations manager at Sydney’s Taronga Zoo, says fatal attacks by great whites on humans are extremely rare.

Rarer still, according to Mr West said, was the presence of two sharks and that Mr Peterson was eaten following the initial attack.

But Mr West added that it is not unusual for sharks to come so close to shore and that the possibility of future attacks cannot be ruled out.

Great whites are a protected species in Australian waters with laws prohibiting hunting of the animals, particularly under threat from commercial fishing.

The powerful predators, which can grow up to seven metres in length, are widely regarded as endangered and Australia has signalled its intention to get the great white listed under the Convention on International Trade in Endangered Species at a meeting of member nations in October.