The woman, who was identified as Anna Shurapey, 55, was swimming about 20 metres off Little Congwong Beach, near La Perouse, at dusk on Friday, when the attack occurred.

Fisheries experts said a juvenile shark of between 2.7 and 3.2 metres in length was responsible, based on photos of the wounds.

It was the first shark attack in Botany Bay in decades, and came exactly 12 months after a shark was filmed cruising close to shore at Brighton-Le-Sands (see video below).

Bayside Council mayor Bill  Saravinovski issued a warning for swimmers in Botany Bay to stay inside netted enclosures.

The woman suffered deep lacerations and puncture marks to her left leg and foot, and was bleeding heavily as she made her way to shore and was assisted by fishermen.

She received first aid on the beach until ambulance paramedics arrived and treated her before she was taken to St George Hospital in a stable condition.

Botany resident Sue Draper, who was among those who assisted, told Fairfax Media she was glad she had completed first aid training.

Ms Draper was supervising her son's youth group paddling in the shallows when she spotted a man struggling to lift a woman out of the water.

When he waved his arms, she and another leader of the group ran to help.

“We saw red in the water… I thought that didn’t look good,” Ms Draper said.

“My friend Alison wrapped a towel around [the woman’s] foot, and with another youth leader, we put her arms around our necks, and did a chair lift to carry her out of the water,” she said.

“It wasn’t until I sat her on the sand that I realised her leg, from below her knee, was sliced open to her ankle.”

Using two bandages from a first aid kit she had brought for the youth group, Ms Draper wrapped the woman’s leg down to her ankle, before using a towel to bandage the woman’s foot.

The woman’s foot had been “serrated” near the ankle and gashed “open” at the bottom.

“[Blood] was seeping through the bandages and onto me … But I was just trying to keep her calm, taking some nice deep breaths,” Ms Draper said.

The group elevated the injured woman's leg, with other beachgoers offering up their towels as emergency bandages while waiting for paramedics to arrive.

“Bloody hell, she was so lucky,” Ms Draper said.

“I think she was probably in shock, or didn’t realise how badly she was hurt, but she was amazing - very brave.

Senior shark biologist at the Department of Primary Industries, Dr Vic Peddemors, said it was “a reasonably severe bite”, but the victim was in a stable condition and underwent surgery.

Dr Peddemors said, from photos provided by police, ambulance and St George Hospital, the injuries were caused by a Great White Shark, between 2.7 and 3.2 metres long.

Juvenile sharks normally fed on fish, but as they got larger they start including turtles and marine mammals like dolphins and seals in their diet, he said.

Dr Peddemors said, while Great White Shark bites on Sydney beaches were fairly unusual because of the meshing program, an incident at Bondi a couple of years resulted in serious injury.

He said it highlighted the danger of swimming at dawn and dusk, which was a feeding time for some species of sharks.

Randwick Mayor Lindsay Shurey said the victim was “”lucky not to have lost her leg...and survive”.

The council closed beaches in the area after the attack and helicopters combed the area on Saturday looking for the shark.