The State Emergency Service (SES) said there was major flooding up and down the coast and the main route from Sydney to Wollongong was closed in both directions.
The SES has had 6800 calls for help since the trough set in on Friday, with numerous flood rescues early Monday morning.
Australia's weather bureau had warned dangerous conditions would continue into Monday with "life-threatening" flash flooding and damaging winds persisting overnight.
Sydney was set to get rainfall at levels not seen in two decades.
