Residents and landholders are warned not to ingest or come into contact with the water from Wallis Creek East Maitland after a high level of blue-green algae was found.
The Hunter Regional Algal Committee issued a red alert on Friday after samples were found to contain a large number of algae, which pose a potential health risk to people and animals.
If consumed, the particular type of algae produces toxins that could cause gastroenteritis in humans as well as skin and eye irritation on contact.
Residents are warned that boiling the water does not kill the toxins and they should avoid any water that appears bright green; where there is green scum, clumps or specs; or has a distinct earthy or musty smell.
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