Ongoing bushfires have incinerated most of the forests in Sydney's main catchment, leaving water reserves exposed to significant contamination in the event of a major downpour.

The Green Wattle Creek fire has burnt about 120,000 hectares around Lake Burragorang, which supplies about 80 per cent of the city's water behind Warragamba Dam. The blaze has a perimeter of 560 kilometres, the Rural Fire Service said.

RFS crews watch from High Range on December 1 as the Green Wattle Creek fire began its surge towards Lake Burragorang, Sydney's main reservoir behind Warragamba dam. Credit:Wolter Peeters

The resulting bushfire ash contains organic material and concentrated nutrients such as phosphorous and nitrogen. While the ash acts like a sponge, if sufficient rain falls in a short time that material could be pushed into the dam, triggering a range of water treatment challenges.

"The worst case scenario would be a very intense rainfall event - 50-60 or maybe 100mm – falling within a few hours," said Stefan Doerr, a researcher at UK's Swansea University, who has studied fires in Sydney's catchments.