SWIMMERS have been warned to keep out of Keepit Dam with a red alert issued for toxic blue-green algae.
WaterNSW advises people should not undertake recreational activities where they may come into direct contact with the water, and stock and domestic users should seek alternate supplies.
"The species of blue-green algae identified are potentially toxic and may cause gastroenteritis in humans if consumed and skin and eye irritations after contact," a WaterNSW statement said.
"Boiling the water does not remove algal toxins.
"People are advised not to enter the water, not to drink untreated water or bathe in water drawn from the dam while this red alert level warning is in place."
Town water supplies are still safe to drink.
Livestock owners have been warned to check stock water supplies for blue-green algae and remove stock from foreshores where surface scum is visible or blue-green algae is suspected.
There's no alert in place for the Namoi River below the storage, but it is recommended livestock and domestic water users check the river water for signs of algae and find alternate water is algae bloom is suspected.
Blue-green algae usually appears as green paint-like scums on the water, near the edges, or as greenish clumps throughout the water, a WaterNSW statement said.
"It makes the water appear dirty, green or discoloured and generally has a strong musty or earthy odour," it said.
"People should not eat fish, mussels or crayfish from red level warning areas.
"Blue-green algae occur naturally and can reproduce quickly in favourable conditions where there is still or slow-flowing water, abundant sunlight and sufficient levels of nutrients."
It's not known how long the algae will remain at high levels but WaterNSW will continue to monitor the dam and the alert will be lifted when the algae dissipates.
Anyone who believes they may have been affected by blue-green algae are advised to seek medical advice.