Christine Armstrong was swimming with a group of friends off a beach in Tathra in New South Wales when she complained and headed back to shore, Police Inspector Jason Edmunds said.
The rest of the group, including her husband, later saw a shark, estimated to be about 11 feet long, swimming near them, causing them to bunch together for safety.
They swam back to the beach club, but found no trace that Armstrong made it back.
Edmunds said that another witness on some nearby rocks spotted a large shark attacking something in the water roughly 150 yards offshore, which he believed to be a swimmer.
The popular beach is now closed for a safety assessment, but an extensive search of the area failed to find Armstrong or signs of the shark.
“The community is in shock, and the husband is taking it hard, as he has been with her since they were kids,” said Edmunds, noting that sightings of large sharks are rare, and he couldn’t remember any other attack in the area.
“Cancer, heart attacks — those are all things we fear but expect, but who gets taken by a shark?”
It is believed the woman was a lifeguard herself, according to a post on the Surf Life Saving New South Wales website. The other lifeguards she was swimming with are receiving counseling, the post said.