Due to the torrential rains of the last few hours, the Warragamba Dam, in southwestern Sydney, began to overflow in the early hours of the morning, well ahead of the authorities' forecasts.
In Camden, a suburb of the city, emergency services rescued 29 people from the floods and were called 1,400 times in a single day.
Thousands of Sydney residents have been ordered to leave their homes due to bad weather.
In March, floods from severe thunderstorms devastated western Sydney, killing 20 people.