Greater Sydney is expected to receive up to 50mm today alone.
A strong wing warning is also in place for the Macquarie Coast, Hunter Coast, Sydney Coast and Illawarra Coast, and flood warnings for a number of inland rivers including the upper McIntyre, Peel and Macquarie river to Bathurst.
The NSW State Emergency Service (NSW SES) says residents should be prepared for the wild weather.
Heavy rain which may lead to flash flooding is forecast to develop in the Central West Slopes and Plains and eastern parts of the Upper Western districts this afternoon, easing later in the evening.
Rainfalls of 60-80mm are forecast, with heavier falls possible if thunderstorms develop, the bureau said.
Residents in the Mid North Coast and northern parts of the Hunter late are being warned to prepare for possible flash flooding this afternoon. Between 80-120mm of rain could fall in these areas.
NSW SES Assistant Commissioner Nicole Hogan recent rains had added to the risk of flash flooding this weekend.
“Our catchments are very wet, and our dams are full, so it will not take a lot for floods to occur,” Ms Hogan said.
“We need people to be mindful of their flood risks and be prepared.”
Flood watches have already been issued for the Upper Macintyre, Gwydir, Peel, Castlereagh, Belubula, and Macquarie to Bathurst.
“In the event flooding does occur, there is a very real chance that roads can be impacted,” Ms Hogan said.
Hail and strong winds are also expected.
It comes after Coffs Harbour only recently was hit by a freak storm which caused extensive damage to homes and businesses.
“Some of our communities, including Coffs Harbour, are still recovering from previous severe weather,” Ms Hogan said.
“It is important you do what you can to protect your homes before the weather hits.
NSW SES urged people to avoid driving, walking or riding through floodwaters.
Residents are also advised to prepare their homes for the storm including moving cars away from trees and powerlines, tying down loose items in backyards and clearing gutters.