WaterNSW has issued a red alert for high levels of potentially toxic blue-green algae in the Hunter River upstream of Glenbawn Dam, from Belltrees to the upper reaches of the lake.
The red alert, which does not apply to Lake Glenbawn, applies only to untreated water and will remain in place until monitoring and test results confirm the risk is sufficiently diminished.
WaterNSW is advising people not to consume untreated water from the affected area and prevent pets and livestock from drinking it.
The agency says livestock owners should check water supplies and remove stock from foreshores where surface scum is visible or blue-green algae is suspected.
People are being urged to avoid recreational activities such as swimming, water skiing, canoeing and any other pastimes that bring them into contact with the water until the red alert warning is lifted.
Blue-green algae can cause gastroenteritis if consumed, and contact can cause skin and eye irritation.
Consumption of water containing algal toxins may cause liver damage and other health problems. Boiling the water does not remove algal toxins.
"People who suspect they have been affected by blue-green algae should seek medical advice," a WaterNSW statement said.
"People should not eat mussels or crayfish from red alert warning areas. A precautionary approach to eating fish from red alert warning areas is advised. Any fish caught should be cleaned and washed thoroughly in uncontaminated water; the internal organs should not be eaten. Avoiding fishing during a bloom is the best way to minimise risk."
