The Sydney basin has been pummelled with the heaviest rainfall in up to two decades with the Warragamba Dam at 61.8 per cent capacity. (AAP Video)

The quality of Sydney’s largest water supply has declined after recent severe weather events.

Heavy rainfall has washed a plume of bushfire ash into Warragamba Dam, causing contamination that has led water authorities to cut the dam’s supply from almost all of Sydney “as a precaution”.

While Sydneysiders yesterday rejoiced at the announcement that rain had caused water levels to jump 33.4 per cent in a week, the truth about the deluge is more complicated than a dam more than half full.

Aerial footage of Warragamba Dam on Sydney’s southwestern fringe shows large brown streaks in the water leading to the dam wall. Other parts of the catchment look filled with dense sediment.

WaterNSW today confirmed the water quality in the heritage-listed gravity damn had been affected by bushfires and heavy rainfall.

WaterNSW said in a statement the confluence of the two severe weather events — 320,000 hectares of the catchment around Warragamba being burned in bushfires, and heavy rainfall causing erosion and flooding — had pushed “increased levels of debris” and ash into the catchment.

Almost all of Greater Sydney stopped being supplied water from Warragamba Dam from Sunday night, as a “precautionary measure”, a spokesman from WaterNSW told news.com.au.

The townships of Warragamba and Orchard Hills are still being pumped water from the dam, where safe water is being taken from 13m below an ash plume washed into the water in recent storms.

Warragamba Dam is not presently supplying water to the remainder of Sydney households, a spokeswoman for Sydney Water confirmed.

The spokesman for WaterNSW told news.com.au “specialist scientists are working on modelling for the future after what’s happened (to Warragamba) due to the fires”.

It is not clear when the water quality will improve, but investigations continue.

“The rainfall in the catchment is not unprecedented in the catchment,” the spokesman said. “The area of the catchment ravaged by fire is unprecedented.”

“Sediment, ash and debris is now clearly visible on the surface of Lake Burragorang,” Water NSW said in a statement.

WaterNSW established a dedicated team to monitor the dam’s systems and water quality as the debris and ash reached the dam wall. The team is linked with Sydney Water, which supplies the city’s drinking water, and NSW Health.

WaterNSW chief executive officer David Harris said he had complete confidence in the team’s ability to work through the ash contamination.

“Raw water quality at Warragamba is improving, however more inflows may cause further deterioration in water quality at the dam wall,” Mr Harris said.

Water NSW said Warragamba was still a viable water source as the sediment was “staying in the upper section of the water column”.

Water NSW said it planned to establish a permanent team to monitor water quality.

It has also installed silt curtains at three locations on Lake Burragorang to limit the amount of ash and debris.