A red alert warning for blue-green algae has been issued for Warren weir, four kilometres south-east of Warren.
WaterNSW is advising that contact with affected water poses a health threat to people, livestock and pets.
He said town water supplies remained unaffected and safe to drink.
"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," the spokesman said.
"It is not possible to predict how long the algae will remain at high levels.
"Regular monitoring will continue and the alert will be lifted as soon as the high levels of algae dissipate."
The spokesman said blue-green algae can cause gastroenteritis in humans if consumed and skin and eye irritations after contact.
"Boiling the water does not remove algal toxins," he said.
"People who believe they may have been affected by blue-green algae are advised to seek medical advice," its spokesman said.
WaterNSW issued a red alert for blue-green algae at Burrendong Dam on November 26.
Visit www.waternsw.com.au/water-quality/algae or call 1800 999 457 for updates.