Ash from bushfires burning across NSW has washed up on Sydney's beaches and there are concerns it could end up in drinking water supplies.
Marine scientists are warning ash could contaminate NSW's drinking water supply, as Sydney's beaches turn black with bushfire debris.
As dozens of fires continue to burn across NSW, waves of burnt matter washed up on many Sydney beaches over the weekend.
The black water was enough to deter most swimmers, but marine ecologist Emma Johnston was among those brave enough to take a dip at Coogee beach for surf life-saving training.
"Everyone was covered in ash, it was all through our bathers, all through our eyes and ears," she said.
But Professor Johnston is more concerned about the effect on drinking water supplies when rain washes the ash into catchments.
"It’s not the saltwater places where people need to be worried, it’s when you're drinking fresh water that’s contaminated that you have real human health issues."
She said the biggest danger is that the ash could cause large algal blooms in drinking water catchments.
The huge amount of burnt vegetation that has landed in the ocean could also lead to mass fish deaths.
"With that high density of particulates, you can get the clogging of fish gills or filter feeders might have trouble feeding," Professor Johnston said.
"A second, more indirect effect is that can trigger an algal bloom. And if you get a really, really large algal bloom, when that starts to decompose it uses up all the oxygen and can cause fish kills from low oxygen environments."
Firefighters used the more mild conditions on the weekend to do critical back-burning and containment work ahead of Tuesday when the temperature is forecast to hit 40 degrees Celsius in some parts of the state.