There is concern that water catchments across NSW could be contaminated with flame retardant chemicals from the bushfire crisis.

Thousands of tonnes of the pink substance have been dropped across fire-affected areas, and experts says some of it may have mixed into the state's drinking water supply, according to the Sydney Morning Herald.

The retardant is used to cover bushland and vegetation to prevent it igniting, slowing the spread of fires.

The non-toxic substance is mostly fertiliser, but that could contain a harmful bacteria not suitable for consumption.

There are also chemicals in the retardants that could pose a risk to aquatic life.

“If fire retardant is applied near waterways or is washed into these areas with rain, there is the potential for it to directly affect fish and other aquatic creatures and indirectly by promoting algal blooms by increasing nutrients in water,” associate professor Tina Bell from the University of Sydney told the newspaper.

NSW Health warns that if drinking water tastes, looks or smells unusual, it should not be consumed by humans or animals.