But the risk of silt entering catchment areas will significantly increase when the drought eventually breaks and "burnt and barren" landscapes are soaked with rain.

A WaterNSW spokesman on Friday said a 45,000-hectare blaze at Green Wattle Creek - 15 kilometres southwest of Warragamba Dam in the Blue Mountains National Park - wouldn't affect Sydney's water quality.

There are also smaller fires to the north at Red Ridge Fire Trail and west of the dam at Kowmung River.

"There's no threat to that (water quality) as yet - we're just in a bit of a holding pattern at the moment," the WaterNSW spokesman told AAP.

However, the corporation and the NSW Rural Fire Service are planning ahead to prevent siltation - the contamination of water with fine particles or bushfire remnants - when the area experiences significant rainfall.

Authorities have deployed booms and curtains to prevent potentially contaminated inflows entering dams, while more than 1100ha of backburning and ridge-top burning around Warragamba has taken place.

Dam operators are also able to supply Sydney Water with water from different areas and different storage depths to ensure quality.

"When the drought breaks, whenever that is, we know we're going to get some rain, and with burnt and barren landscapes, we know we're going to get siltation, runoff, erosion," RFS commissioner Shane Fitzsimmons told reporters on Friday.

"That's very front and centre - we're working together with land management agencies, environmental authorities and the relevant water authorities.

"We know we're doing all we can to limit the intensity of fire that gets closer and closer to those catchment areas."

Australian Associated Press