Water NSW chief executive David Harris says the corporation is working to minimise the contamination of Warragamba Dam after almost 320,00 hectares of the catchment was burnt.

Two booms - also known as silt curtains - have been placed upstream of the dam to catch silt before it reaches the dam itself.

"We do expect there will be some ash and debris run-off into Warragamba Dam during the event," Mr Harris told ABC TV on Friday.

The chief executive said the catchment area had received about 15 millimetres on average over the past few days with some parts receiving upwards of 70mm.

Water NSW modelling predicts rainfall could reach 130mm over the next 72 hours which could raise the dam level by four per cent. It currently sits at 42.8 per cent.

"It could be a lot more than that - but that's a very welcome improvement," Mr Harris said.

Water is currently being taken from 30 metres below the dam's surface with contaminated water sitting at the top at a higher temperature.

Mr Harris hopes bales can be installed further away from the dam in the coming weeks to stop bank erosion and slow down water flowing into the storage.

Water NSW anticipates at least 75mm of rain is required in the catchment before significant run-off is generated for inflows.

"Managing water quality in our dam storages is what we as an organisation do and our water quality scientists are highly-regarded experts," Mr Harris said in a statement.

He said the full impact of rain on dam levels will become clearer over the coming days.

Water NSW will also be looking at Brogo Dam on the state's South Coast to see if significant rainfall will improve the dam's water levels.

The Bureau of Meteorology says the intense coastal trough off northern NSW has delivered close to 300 millimetres in some locations already and forecasts about 200mm will fall over the catchment area across the weekend.

"The rain is making its way into the (Warragamba) catchment and it should see quite a bit of rain over the next few days," bureau acting NSW manager Jane Golding told reporters on Friday.

Charles Sturt University Professor Max Finlayson says minimising the impact on vital water supplies and waterways needs to be a priority to avoid water quality issues, toxic algal blooms and further fish kills.

Prof Finlayson told AAP on Thursday that sediment traps, geo-fabrics, logs or straw bale barriers were key tools.

But, he acknowledged, the scale of the bushfire crisis could make implementing such strategies difficult in the shorter-term.

Australian Associated Press