Intense showers are forecast for metropolitan Sydney, the Illawarra and parts of the Central Tablelands over the weekend, with the possibility of flash flooding.
The Bureau of Meteorology warned of six-hourly rainfall totals of between 80 and 150mm across Saturday and Sunday, with some areas forecast to receive a month's worth of rain over the weekend.
The BOM said on Friday afternoon the system may develop into a low on Sunday or Monday, prolonging the persistent rain into next week.
Heavy falls across the weekend could to lead to rising river levels as the deluge hits multiple already-saturated catchments.
"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."
Ms Golding said the deluge could lead to flash flooding and landslips.
"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.
Areas at risk include Newcastle, the Central Coast, Lake Macquarie, the Upper Coxs, Colo, Macdonald, Woronora, Patterson, Williams and Lower Hunter rivers.
Also at risk are 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.
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