A month's rain to lash Sydney overnight

A month's worth of rain is expected to drench Sydney overnight, with motorists advised to avoid driving.

A whopping 87mm of rain is predicted to hit the city by midnight on Wednesday, almost 7mm above the average monthly rainfall for August of 80.6mm.

Heavy rainfall and flash flooding are expected to hammer parts of Sydney and the Central Coast, sparking warnings from the NSW SES for motorists to avoid getting behind the wheel.

"Stay off the roads. If you do have to drive this afternoon ... don't drive or ride or walk through floodwaters," SES spokesman Ashley Sullivan said.

Some areas are expected to be hit by more than 100m of rain, the Bureau of Meteorology told AAP.

"We are expecting a few stations in Sydney and the Central Coast to reach or exceed 100mm but the bulk will receive 40-90mm," BOM spokeswoman Rosemary Barr said.

The severe weather warning covers the eastern half of greater Sydney and the Central Coast as a low pressure system forms across the state's coast before moving east on Thursday morning.

By Wednesday morning, just over 97.2mm of rain had fallen in Sydney since the start of August, well above the total monthly average.