A young woman has been mauled by a shark in Sydney Harbour, with a vet who witnessed the horror unfold credited with saving her life.

Emergency services rushed to a private wharf in in Elizabeth Bay at 7.45pm on Monday to find the woman, aged in her 20s or early 30s, with a severe bite to her right leg and suffering ‘major blood loss’.

A vet who was passing by had already applied a tourniquet to stem the bleeding from the woman’s leg and may have saved her life – but there are fears she could lose her leg.

‘She had serious bleeding. Her injuries are severe,’ a NSW Ambulance spokesman told Daily Mail Australia.