Photo: Xinhua

 Authorities have raised concerns about severe bushfires near Sydney's main drinking water catchment, suggesting that a build of ash could soon contaminate the precious resource.

On Friday, fire crews continued to battle blazes which have almost entirely surrounded Lake Burragorang, which is responsible for supplying roughly 80 percent of Sydney's drinking water via Warragamba Dam.

Professor of civil and environmental engineering at the University of New South Wales Stuart Khan told the Sydney Morning Herald that there were two main risks to the water supply.

Firstly the fire would cause damage to infrastructure such as pumping stations and pipes, and secondly ash would wash into and pollute the water supply itself.

So far, largely due to preventative measures by firefighters, vital infrastructure remains untouched, while due to a lack of rainfall, ash and other organic matter has been piling up rather than washing into the lake.

Khan predicted that a big enough downpour of rain, between 100mm to 200mm, could wash that large build up of material into the water, causing serious issues.

That amount of rain is not predicted for several months, however WaterNSW has taken precautionary measures, including deploying floating booms and curtains to block ash from entering the supply, while monitoring water quality closely.

Conditions on the fire grounds are expected to worsen once again next week, with "extreme heatwave conditions" predicted to hit the area on Monday and Tuesday.