A RED alert for blue-green algae has been issued for parts of the Wakool River between Stoney Crossing and Kyalite.
WaterNSW said the warning applied to untreated water and that people should not consume untreated water from the affected section of the Wakool River.
The public should also prevent pets and livestock from drinking this water.
WaterNSW said it advised people to avoid recreational activities such as swimming, water skiing, canoeing and any other activity that brought them into contact with the river until it lifted the red alert.
Water containing toxic blue-green algae can cause gastroenteritis, while contact can lead to skin and eye irritations.
Drinking water containing algal toxins may cause liver damage and other health problems. Boiling the water does not remove algal toxins.
Anybody suspecting they have been affected by blue-green algae should seek medical advice.
While the latest advice has not ruled out eating fish caught from the affected portion of Wakool River, any fish caught should be cleaned and washed using uncontaminated water. The internal organs should not be eaten.
WaterNSW said the best way to minimise risk was to avoid fishing during the bloom.
Blue-green algae is naturally occurring and can reproduce quickly in still or slow-flowing water when there is abundant sunlight and sufficient nutrients.
It usually appears as green paint-like scums on the water, near the edges, or as greenish clumps throughout the water. The water can appear dirty, green or discoloured and a strong musty or earthy odour may be detected.
