Curtains and booms to trap silt and ash from bushfires have been installed in the Warragamba Dam, as concerns grow that rain could lead to contamination of the water.

Key points: There are fears bushfire debris could end up in Warragamba Dam when rain arrives later in the week

There are fears bushfire debris could end up in Warragamba Dam when rain arrives later in the week Warragamba Dam supplies water to more than 5 million people living in Sydney and the lower Blue Mountains

Warragamba Dam supplies water to more than 5 million people living in Sydney and the lower Blue Mountains Experts say chlorine may need to be added to regional dams to dilute ash and keep water drinkable

Bushfires have surrounded almost all of the perimeter of the dam, with Water NSW saying more than 300,000 hectares of catchment land has been burned.

Now that rain is forecast for the end of the week, run-off full of ash and other debris could end up in Warragamba Dam, which is Sydney's main water supply.

"Once you have a fire go through a drinking-water catchment, the ash that's left on the ground and the ground itself becomes more repelling of water," Stuart Khan, a water expert from the University of NSW, said.

"So even small rainfall events instead of soaking in do tend to run off after a fire.

"I think it's a risk that we need to take very seriously."

NSW Water Minister Melinda Pavey said booms had been installed in the dam to try and catch some of the ash.

There are fears rain could lead to contamination of the water at Warragamba Dam. ( ABC TV )

There are two booms, one about 500 metres from the dam wall and another 9 kilometres upstream, with silt curtains attached to catch the run-off.

The Government says it has another 1,000 metres of silt curtains on hand in case they are needed for either the Nepean or Tallowa dams.

"It is something we've been worried about since the fires and we've had a lot of energy and a lot of focus on ensuring we protect Sydney's catchment," Ms Pavey said.

Professor Khan said the water coming out of the tap would still be clean and drinkable, but the silt and ash would create a challenge for the Prospect Water Filtration Plant.

"The immediate problems really are problems for the water treatment processes," he said.

"Water treatment plants are designed to remove contaminants like that from the water, but when we have very large quantities of sediment they will really struggle."

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The Warragamba Dam is currently at 43.7 per cent capacity, which means rain is desperately needed, but there is less opportunity for ash and other debris to be diluted.

Professor Khan said smaller regional dams might need more intervention to keep water drinkable, like the addition of chlorine.

Some concerns have also been raised about the prospect of fire retardant used during bushfires running into waterways.

Ms Pavey said the retardants had not been used near Warragamba Dam, and the NSW Rural Fire Service said it had a rule that retardant drops happened at least 100 metres from any waterways.