Authorities have reconnected drinking water in Greater Sydney to Warragamba Dam after it was cut off due to bushfire contamination caused by last week's flooding.

Key points: Sediment, ash and debris from burnt bushland was seen floating on the surface of Lake Burragorang near the dam wall

Sediment, ash and debris from burnt bushland was seen floating on the surface of Lake Burragorang near the dam wall A water experts said recent replenishment of other dams means Sydney's water supply was secured for months, even without the dam

A water experts said recent replenishment of other dams means Sydney's water supply was secured for months, even without the dam Level two water restrictions remain in place in Sydney

Water NSW made a decision earlier this week to stop using Warragamba Dam as a water source due to a torrent of fire and flood debris which ran into the catchment.

For days, Sydney and its outer suburbs were drawing their drinking water from Prospect Reservoir.

Last week, a torrential downpour, which caused flash flooding and power outages across NSW, filled the dam's water storage from 41 to 75 per cent.

Sediment, ash and debris from 320,000 hectares of burnt bushland — almost the entire perimeter of the dam — was seen floating on the surface of Lake Burragorang, where the dam wall was located.

Fire and flood debris seen floating on the surface of Lake Burragorang this week. ( ABC News )

Water NSW said Warragamba Dam was being returned to "being the main source of untreated water supply to Sydney as of this weekend".

"Ongoing monitoring results confirmed confidence in the quality of water available from Warragamba," a spokesperson said.

Water NSW chief, David Harris, said an incident response team was established and it was working with Sydney Water and NSW Health to develop monitoring and contingency plans.

"Raw water quality at Warragamba is improving, however, more inflows may cause further deterioration in water quality at the dam wall," he said.

He said a "range of precautionary measures on-site" were taken, including deployment of additional floating booms to capture silt and ash from bushfires.

An area of the Warragamba Dam hit with runoff from the bushfire zone. ( ABC News: Brendan Esposito )

Western Sydney University water expert, Ian Wright, said there "must have been sufficient concern to stop supply" but he could not see any "adverse effects" from the decision.

"The real question, I guess, is how long the Warragamba [would have] remained switch off," Dr Wright said.

"The good thing is that the other catchment dams were overflowing from the rains [which] have not had bushfires near their catchments."

Last week's rainfall saw the Nepean and Tallowa dams completely topped-up.

"Combined they make up about 20 per cent of Sydney's water supply so we can last a long time — months, possibly even a year."

Ian Wright said bushfires have compromised water catchments across NSW. ( ABC News: Billy Cooper )

Water NSW spokesperson Tony Webber said there was no visible impact to Sydney's drinking supply and water was being drawn from Prospect Reservoir because it was the best quality available.

"It is important to note that this issue only concerns untreated water in dam storages at the beginning of the supply chain and has no impact on the quality of treated water," Mr Webber said.

Level two water restrictions remain in place in Sydney.