Warragamba Dam began spilling in the early hours of this morning following heavy rain and thunderstorms across Sydney this week.

At 3.25am, the dam in Sydney's outer south-west was reported to be overflowing and may continue spilling for some time.

The statement from WaterNSW comes days after Sydney experienced it's wettest September day since 1879.

It is the second time in four weeks Warragamba Dam has been reported to be overflowing due to wild weather conditions.

WaterNSW has urged people to stay away from potential hazards, such as fast-flowing and deep water near waterways and floodplains.

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