Water NSW has issued a recreational red alert warning for the Williams River at Seaham Weir and Clarence Town due to blue-green algae blooms.
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Warning signs have been put in place while the high levels are present, the authority said, but drinking water is unaffected.
The red alert means people should not swim or undertake other activities that would bring them into direct contact with the water. The algae may also threaten livestock and pets who come into contact with it.
Anyone affected by the algae is advised to seek medical advice.
“People shot not eat mussels or crayfish from red alert warning areas,” Water NSW said in a statement. “Any fish caught should be cleaned and washed thoroughly in uncontaminated water and any internal organs disposed of before consumption.”
Blue-green algae occurs naturally and reproduces quickly in still or slow-flowing water with abundant sunlight and sufficient nutrient levels.
Water NSW said it is not possible to predict how long the high level will remain but monitoring will continue until the blooms subside and the alert can be lifted.