The low-lying areas around the rivers Nepean and Hawkesbury are particularly hard hit by the flood. But citizens in other places are also being advised to prepare for a possible evacuation.
The heavy rainfall around Sydney has been going on for days. In the whole month of July, four times as much rain fell as normal, Australian media report. Weather conditions are expected to improve again on Tuesday.
New South Wales emergency services say they rescued about 20 people in the past 12 hours. In most cases, it concerned people who got stuck with their cars on the flooded roads.