Dam levels across greater Sydney have risen by more than 20 percentage points amid heavy weekend rainfall, with enough water for as much as 150,000 Olympic-size swimming pools flowing into Warragamba Dam.
Sydney, the Central Coast and the Blue Mountains were soaked after receiving between 200mm and 400mm of rain from 9am Friday to 5pm Sunday, with additional rain expected on Monday and through the week.
The Bureau of Meteorology suggested on Sunday that Sydney had not experienced such rainfall since before the new millennium.
Water NSW data on Monday morning showed that greater Sydney dam levels were at 64.2 per cent, up from 41.9 per cent in seven days.
Warragamba Dam recouped a year's worth of water in one weekend, rising 17.7 percentage points to sit at 60.7 per cent at 10.30am on Monday. Some 360,000 megalitres of water flowed into the dam - almost 150,000 swimming pools.
The dam this time last year was 61.4 per cent full.
Water NSW in a statement on Sunday said the ash and debris generated by recent bushfires around the Warragamba area would not affect water quality.
Two silt curtains are in place to intercept ash run-off.
"While it is likely some ash and debris has been washed into the upper reaches of the Warragamba system, there is no impact on the water quality being supplied for treatment. Any surface debris is being avoided by extracting water from 30m below the surface as a precaution," WaterNSW said in a statement.
Prospect Dam is more than 90 per cent full, while Woronora Dam to Sydney's south is almost 60 per cent full, jumping 25 percentage points.
The Bureau of Meteorology has forecast more severe weather conditions on Monday with heavy rains, strong winds and damaging surf possible along the entirety of the NSW coast and inland southern NSW.
Flash flooding remains a possibility in several NSW regions, including Sydney.