More than 40 people have been rescued after heavy rains hit Sydney, causing flash floods that have also knocked out power.

The NSW State Emergency Service (NSW SES) said it had conducted 42 flood rescue operations in central and western Sydney from Thursday to Friday.

The Sydney metropolitan area was called to help after some suburbs received more than 100 millimetres of rain in three hours, the SES said.

The flash flooding affected major roads and caused damage to electrical infrastructure and property.

The Bureau of Meteorology (BOM) has forecast thunderstorms and up to 70 millimetres of rain in Sydney for most of Friday.