A surfer who went missing at an Australian beach is now believed to have been the victim of a shark attack, according to authorities.

Just after 7 p.m. local time on Thursday, a 28-year-old man from Port Lincoln, a city on the Eyre Peninsula, was surfing near Granites Beach, south of Streaky Bay, when witnesses reported seeing a shark attack the man, South Australia Police wrote in a press release early Friday.

Authorities say there's "been no sign of" a local surfer seen being attacked by a shark off South Australia's coast.

Surfing on South Australia's west coast is now too dangerous because of a surge in the population of great white sharks, a friend of a shark attack victim says.