Water NSW advised people not to enter the water, drink untreated water, or bathe in water drawn from the dam in a warning issued this week.

The red alert came after monitoring by Water NSW experts indicated that concentrations of blue-green algae have increased to dangerous levels in the dam again.

There is also an active amber alert for blue-green algae in the Macintyre River at Lake Inverell.

Town water supply for the community of Inverell is unaffected by the algal bloom.

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The species of algae, called cyanobacteria, is potentially toxic and can cause gastroenteritis in humans after consumed, and can cause skin and eye irritations after contact.

Water NSW advises water users to visually monitor downstream water supplies for algae scums. Algae usually appear as green paint-like scums on the water, near the edges, or as greenish clumps throughout the water. It makes the water appear dirty, green or discoloured and generally has a strong musty or earthy odour.

Water NSW also advised people not to eat fish mussels or crayfish from warning areas. Contact with the water is also a threat to livestock and pets. Boiling the water won't make it safe.

A red alert is the highest level of warning Water NSW can issue.

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.

People who believe they may have been affected by blue-green algae are advised to seek medical advice.

Copeton is currently 41.6 per cent full, according to Water NSW.

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