Passers-by save Sydney snorkeller

A woman snorkelling off a Sydney beach has been rescued by passers-by who noticed the 23-year-old in trouble.

The bystanders then performed CPR on the woman, who began breathing on her own as emergency crews arrived on Monday at Little Bay, in the city's southeast, a NSW Ambulance spokeswoman says.

The woman has been taken to St Vincent's Hospital in a serious but stable condition.