WaterNSW is warning of "potentially toxic" blue-green algae in the Macquarie River downstream of Burrendong Dam.
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It has issued a red alert after the algae were found in the "immediate vicinity" of the dam wall.
A WaterNSW spokesman reports that "algae can move".
But a monitoring station at Wellington had not registered at the red alert level, he said on Thursday.
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WaterNSW is advising of possible health impacts of "coming into direct contact" with affected water.
"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," the spokesman said.
"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."
The spokesman urged people using river water for stock and domestic purposes to access alternate water supplies and move stock from "foreshores where surface scum is visible or blue-green algae are suspected".
WaterNSW reports that boiling water does not remove algal toxins but treated town water supplies are unaffected and safe to drink.
"Blue-green algae usually appear as green paint-like scums on the water, near the edges, or as greenish clumps throughout the water," the spokesman said.
"It makes the water appear dirty, green or discoloured and generally has a strong musty or earthy odour.
"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."
For more information visit www.waternsw.com.au/water-quality/algae or call 1800 999 457.
People who believe they may have been affected by blue-green algae are advised to seek medical advice.