Water bureaucrats have also been working "around the clock" to protect water supply assets from flames that came within kilometres of drought-depleted dams. The Green Wattle Creek and Ruined Castle fires surrounding Lake Burragorang, which supplies about 80 per cent of Sydney’s water through Warragamba Dam, have burnt more than 223,000 hectares, nearly doubling in size in the past fortnight. In early December, Rural Fire Service firefighters conducted strategic backburns around Warragamba as the Green Wattle Fire took hold on the eastern side of Lake Burragorang. Loading Replay Replay video Play video Play video The lake and dam have now been almost completely encircled by bushfire, with only a "small portion" on the northern side of the lake currently untouched by fire, RFS spokesman Ben Shepherd said.

The impact of the spread of fires in the past two weeks affecting the Warragamba catchment had been "really extreme", said Professor Stuart Khan, a professor of civil and environmental engineering at the University of NSW. "The majority of the perimeter of Lake Burragorang has been impacted, and Wollondilly and Coxs rivers, with significant quantities of ash flowing," he said. There were two risks to water supply, said Professor Khan, an expert on water contamination. The first was that fire would destroy pumping stations and damage pipes, which so far hadn't happened, and the second was threat of a large downpour causing ash to run off and pollute the water supply. Even before the fires, the lack of rain had meant organic material which could pollute water had been building up in greater concentrations. From previous experience and modelling, he said if there was a big downpour - in excess of 100mm to 200mm of rain, causing run off for the first time in two years - that could cause serious problems. WaterNSW has since deployed floating booms and curtains across the Warragamba catchment to serve as a barrier to block ash from filtering into the untreated part of the water supply, while water quality scientists are monitoring the dam using sophisticated, real-time technology.

"The current priority is to protect against ash or debris being washed into the storage following a rain event," the WaterNSW spokesman said. Water Minister Melinda Pavey said the NSW water agencies had been "working around the clock to ensure that our towns have access to water." "Our assets across the state remain intact and are still supplying water to towns affected by these severe fires," Ms Pavey said. Loading But Professor Khan said the combination of fires and drought had pushed water managers "out of their experience and comfort zone".

A spokesman for WaterNSW said the fact there had been no significant damage to assets in the Blue Mountains and Warragamba Dam areas was a testament to the firefighting efforts of the RFS. Bureau of Meteorology forecaster Jiwon Park said while there have been benign conditions over Christmas, a heatwave will bring temperatures in the 40s to parts of Sydney over the weekend before a southerly change hits the state early next week. "Ahead of the southerly the temperatures will be rising," he said. Loading Bureau of Meteorology duty forecaster Rose Barr told AAP some areas were forecast to reach "extreme heatwave conditions".