More areas of Sydney have been ordered to evacuate because of a major flooding risk, as heavy rains continue to batter Australia's New South Wales state.

The authorities say anyone living in low-lying areas of the city must leave.

At Sunday's briefing, NSW Premier Gladys Berejiklian warned parts of the state's Mid North Coast were experiencing a "one in 100-year event".

A day earlier, Warragamba Dam, Sydney's main water source, started to overflow for the first time in years.

Dozens of people have been rescued from floodwaters in New South Wales, with officials warning of "life-threatening flash floods".

Major roads remain shut in the area.

Ms Berejiklian said thousands more people in Sydney could be ordered to evacuate.

Hundreds of people have already been gathering at emergency evacuation centres set up across the state.

Meanwhile, state emergencies officials said a number of schools in the affected areas would be closed on Monday. Residents were also asked to work from home.

Up to 100mm (four inches) of rain is forecast to fall in 12 hours over Sydney, and as much as 300mm for the lower Blue Mountains, west of the city.

Torrential rains and powerful winds will likely continue until late next week, with floods not expected to subside until Thursday.

Sydney residents posted pictures on social media of flooded roads and rising waters nears their homes.

Agata Imielska from the Bureau of Meteorology warned of localised intense rainfall and damaging winds, saying the public should be aware of "dangerous conditions" that can change quite quickly.

"If you don't need to travel, if you don't need to head out today, this is the day to stay at home," Ms Imielska was quoted as saying by the Sydney Morning Herald.