Warragamba Dam began to spill this morning as Sydney faces another day of heavy rain.
Water NSW said the catchment began overflowing at 7.30am today and may continue to spill for some time.
Residents are being urged to stay away from waterways and flood plains, including the Hawkesbury and Nepean rivers.
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The Bureau of Meteorology said heavy rain along the southern coast and adjacent ranges was likely to ease later tonight.
However, the downpours may trigger minor flooding to moderate flooding in parts of the Hawkesbury Nepean Valley, St Georges Basin and Moruya and Deua catchments.
"Catchments are relatively wet along much of the NSW coast due to rainfall and isolated flooding in recent weeks," the bureau said in a midday update.
"The weather system may cause flooding across the Flood Watch area where flood warnings are current.
"River levels are elevated; however, flooding is no longer expected for the Bega and Clyde Rivers."
The Hawkesbury River at North Richmond is expected to exceed the minor flood levels at 4.20 metres by this evening.
There are no flood warnings in place for the Upper Nepean however, Menangle Bridge is expected to remain below the minor flood level of 5.2m throughout today.
The Colo River at Putty Road may reach the minor flood level of 2.7m by this evening, with low-lying communities mostly impacted.
Earlier, a flood advice alert was issued for Wamban on the NSW South Coast after downpours of 150mm were recorded across the Moruya and Deua Rivers in the last 48 hours.
"You should stay informed by monitoring warnings issued by NSW SES on their website and Facebook page, listening to your local radio station, and checking the latest weather information from the Bureau of Meteorology online," the SES said in an alert.
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The bureau said the South Coast and Illawarra was forecast to receive more than 100mm of rain over the next 24 hours.
Sydney is expected to record a further 20mm today after recording downpours of 30mm overnight.
"An upper level low over the Central Tablelands is pushing an area of rain into the southeast coast," the BoM said.
"The rain will contract northwards out of the South Coast during the day, and off the Illawarra coast through the afternoon and evening."
The Bureau warned heavy rain may lead to flash flooding in northern parts of the South Coast, southern parts of the Illawarra, eastern parts of the Southern Tablelands and far north-eastern parts of the Snowy Mountains districts this morning and afternoon.
Areas that may be affected include Nowra, Batemans Bay, Moruya Heads, Ulladulla, Narooma and Araluen.
Overnight, the SES attended Dover Heights in Sydney's eastern suburbs after heavy rain caused a sinkhole to open up at a construction site on a residential street.
SES crews remained on scene until 1am, using sandbags and a retaining wall to divert water away from surrounding properties.
The incident was one of more than 270 calls for assistance in which SES volunteers attended in the 24 hours to 8am today.
Assistant Commissioner Nicole Hogan said most calls were for leaking roofs, fallen trees and sandbagging properties in the state's south-west.
"Since the start of the weather event, we've responded to more than 415 incidents across the state and have had about 300 volunteers out helping their communities," Hogan said.
The SES is urging residents to take care as an additional 150mm is expected to fall in the Illawarra and south coast regions later today.
