Southern Rural Water is warning residents to avoid swimming, fishing and coming into contact with water at Melton Reservoir due to a potentially toxic blue- green algae outbreak.
Monitoring has detected an algae bloom, which can be dangerous to humans and animals.
Blue-green algae can cause skin rashes or itchiness; sore eyes, ears and nose; or if swallowed, gastroenteritis, nausea or vomiting.
Water from Melton Reservoir should not be used for drinking, cooking or other domestic uses. Boiling the affected water will not make it safe.
Signs have been placed in the area to warn people of the hazards.
Southern Rural Water will continue to monitor the situation and will remove warning signs when the water is considered safe.
People who come into direct contact with contaminated water should wash immediately in fresh water or seek medical advice if symptoms persist.