A shark mauled a woman as she swam in Sydney Harbour in a gory attack that left her in hospital with a "serious" leg injury, officials and media said Tuesday.

The predator struck Monday evening as the woman swam off a wharf at Elizabeth Bay, less than two kilometres (1.2 miles) from the iconic Sydney Opera House, police said.

The woman suffered a "serious injury to her right leg", New South Wales police said in a statement.

It was the first shark attack in Sydney harbour since 2009, when an Australian navy diver fought off a bull shark that bit him in the arm and leg in Woolloomooloo Bay.

Neighbours rushed to help the Elizabeth Bay victim, the Sydney Morning Herald said, identifying her as 29-year-old Lauren O'Neill.

It quoted one resident who heard her "soft yell" for help from his window.

"She was trying to climb in and behind her was her leg, which was completely open and full of dark red blood behind her," resident Michael Porter was quoted as saying.

"It was surreal. We have always been worried and known about sharks in the harbour … it's only now that it feels very real."

A vet living nearby applied a tourniquet, the paper said.

"She swam out to the boat and on her way back she got bitten by, I think it was, a bull shark," another witness told local news outlet OnScene Bondi.

"We ran out. My wife's a vet and she basically bandaged it up," said the witness, who was not named.

The woman was in stable condition in intensive care at St Vincent's hospital, a hospital spokesperson said.

She was expected to undergo surgery during the day.

In February 2022, 35-year-old British diving instructor Simon Nellis was devoured off Sydney's ocean beach Little Bay, the first fatal attack in the city since 1963.