A massive amount of rain has fallen in Sydney overnight, causing flash flooding in some areas.

More than 700 calls were made to the State Emergency Service due to severe thunderstorms sweeping across New South Wales.

The biggest falls were in Sydney's north, with 88 millimetres of rain falling in Hornsby and 73 millimetres coming down on Mona Vale in just one hour.

Weather forecaster Chris Webb says Sydney experienced one of the highest rainfall events of the last decade.

"For the city we recorded 65 mills of rainfall in the evening, and that's about a one in five - ten year event," he said.

Mr Webb says more rain is on its way, which could bring more flooding.

"We do expect further showers and possible thunderstorms during the day today, Saturday, and then a further increasing trend in the rainfall and thunderstorm activities tomorrow," he said.

"So still over the weekend, there's still the possibility of further heavy falls."

The State Emergency Service says crews had to rescue people from six cars stranded in floodwaters and assist others trapped in buildings.

Meanwhile, the fire brigade responded to numerous collapsed rooves, including a nightclub in Kings Cross.