Bystanders have been praised for helping save a woman who was savagely mauled by a shark while taking an evening swim in Sydney Harbour on Monday.

The woman, aged in her late 20s, had been in the water off a private wharf in Elizabeth Bay when the horrifying attack unfolded about 7.45pm.

One witness said the woman swam out to a boat, and was bitten on the right leg “by a bull shark” while attempting to return to the jetty.

Bystanders heard a cry for “help” and rushed to the victim’s aid as she tried to make it back out of the water.

Paramedics arrive at the jetty where bystanders were assisting the woman. Photo: TNV screenshot

One resident, Georgia, said her veterinarian wife managed to keep the woman alive by applying a tourniquet until paramedics arrived.

“We heard a shout … we ran out. My wife’s a vet, she basically bandaged it up … Bone’s broken – it’s pretty hectic, actually,” Georgia told TV crews at the scene.

“We got some bandages, we kept her warm. My wife bandaged her up to stop the bleeding.”

Georgia said the female swimmer “seemed to be OK” despite the terrifying ordeal.

“She was in a lot of shock – so am I,” she said.

“If she got bitten [further out into the water], she wouldn’t have survived.”

The woman had severe injuries and was taken to St Vincent’s Hospital. Photo: TNV screenshot

NSW Ambulance Acting Inspector Brett Simpson praised bystanders who “did an exceptional job looking after this young lady”.

The vet administered first aid on the injured woman, who had severe blood loss from injuries to her lower leg.

She was rushed to St Vincent’s Hospital in a critical condition.

Footage shows blood staining the private jetty’s wooden planks in the aftermath of the incident.

Tweet from @ClrLindaScott

-with AAP