At least three homes have been flooded in Sydney's west as torrential rain brought flash flooding to several parts of the state.
The State Emergency Service (SES) says it responded to 80 calls for help from 6pm on Thursday for flooded homes, leaking roofs or fallen trees as heavy rain battered NSW.
Three homes in Londonderry in the city's west were inundated overnight with about 40cm of water, and there have been reports raw sewerage was swept onto the streets because of blocked drains.
SES crews were called to sandbag areas in the city's southern and northern areas, as well in the Illawarra and state's south coast.
The soggy weather is expected to get worse this weekend, with gusty winds and more rain forecast.
The Bureau of Meteorology on Friday issued a severe weather warning for heavy rainfall in the Illawarra, South Coast, parts of the Southern Tablelands and Snowy Mountains districts.
The weather bureau said it expects the low-pressure system to develop off the east coast late on Friday, and has warned the thunderstorms may lead to flash flooding - with more than 100mm of rain in some areas.
A flood watch was also in place for the Moruya and Bega river valleys in the state's south.
The SES has urged people to prepare for the deluge, and says Sydney is unlikely to escape the wet conditions.
"People need to be prepared for flash flooding, particularly those that live in low-lying streets in the innerwest," SES spokeswoman Becky Gollings told AAP.
"Those streets can go under at the drop of the hat so people need to think about taking different routes or not getting caught out," she said.
The weather bureau has forecast the rain to ease on Sunday, with only a few millimetres of rain expected in Sydney and the south coast regions.