NSW residents have been warned to brace for yet more heavy rainfall, river rises, flooding and possible landslips as a stubborn low forms over the coast.
"We know these are quite dangerous systems," Bureau of Meteorology Manager of Hazard Preparedness and Response Jane Golding told media on Friday.
"They've been known to produce some widespread flooding in the past, and that's certainly on the cards for the next few days."
Showers already falling across the east coast are forecast to increase on Saturday, Sunday and Monday, developing into widespread areas of persistent rain.
"Certainly there'll be a response in the rivers as a result of that rain," Ms Golding said.
On Friday the areas of impact are expected to include Greater Sydney and the Illawarra region, with hundreds of millimetres of rain forecast to fall.
"We're expecting some locations over the next few days, and even tomorrow, could see what they would normally see in the entire month of July all in one day," Ms Golding said.
She said the deluge could lead to flash flooding and landslips.
"We know that the landscape is quite vulnerable at the moment and the water can move very quickly down the slopes and through the waterways," she said.
Flooding is possible for the Hunter, Central Coast, the Greater Sydney region and the South Coast from Saturday, with flood watches in place for catchments between Newcastle and Batemans Bay, including Sydney and the Illawarra.
From Saturday, risk areas include Newcastle, the Central Coast, Lake Macquarie, the Upper Coxs, Colo and Macdonald rivers.
Also at risk are the the Upper and Lower Nepean and Hawkesbury rivers.
"What our hydrologists are seeing is the rain is potentially enough to cause major flooding there again, which will be the third or fourth time in the last two years for those communities," Ms Golding said.
Warnings of flash flooding are in place for the Sydney, Illawarra and Shoalhaven areas and the Cooks and Georges rivers.
The SES warned residents not to expect the same consequences from heavy falls as previously, with saturated catchments likely to produce different results.
"The accumulation of heavy rainfalls over the number of days will have a different effect on the flow of rivers," NSW SES Commissioner Carlene York said.
"A different effect on flash flooding which, by its name, can come very quickly and be very unexpected.
"The main message from the SES is for people to think about their own safety and the safety of their family and friends."
