Red alert issued for blue-green algae at Williams River’s Seaham Weir

newy.com.au – WaterNSW has issued a red alert for high levels of potentially toxic blue-green algae in untreated water at the Williams River at Seaham Weir in the Hunter region on Friday 27 February 2026.

The warning means people should stay out of the water at the affected site and avoid drinking untreated river water, while Hunter Water said its treated drinking water supply remains safe.

WaterNSW said the red alert was issued after sampling conducted by Hunter Water and applied only to untreated water at the identified location. The agency said it would stay in place until monitoring confirmed the risk had eased, adding: “This alert does not affect Hunter Water’s drinking water supplies, which remain safe to drink.”

Residents and visitors were advised not to consume untreated water drawn from the river at Seaham Weir and to prevent pets and livestock from drinking it. WaterNSW also urged people to avoid recreational activities and any other activity that brings them into contact with the water, including swimming and watersports, until the warning is lifted.

The agency said potentially toxic blue-green algae could cause gastroenteritis if consumed, while contact can cause skin and eye irritation. It warned that consuming water containing algal toxins could cause liver damage and other health problems, and said boiling water did not remove algal toxins.

Anyone who suspects they have been affected was advised to seek medical advice.

WaterNSW said people should not eat mussels or crayfish from red alert areas and recommended a precautionary approach to eating fin fish caught there. It said any fish should be cleaned and washed thoroughly in uncontaminated water and the internal organs should not be eaten, adding that avoiding fishing during a bloom was the best way to minimise risk.

The agency said blue-green algae occur naturally and can reproduce quickly in still or slow-flowing water when there is abundant sunlight and sufficient nutrients, and directed people to Hunter Water for information about management of algae risks in nearby treated drinking water supplies. Updates on blooms and red warning areas are available through WaterNSW’s algae information and WaterInsights.