A blue-green algal alert has been issued for Myall Lake, Boolambayte Lake and Bombah Broadwater.
Water sampling was showing algae numbers potentially risky to humans.
Blue-green algae may contain toxins causing severe stomach upsets, nausea and skin irritation in people that cannot be removed by boiling the water.
It appears as green paint-like scum on water, as well as edges or clumps through the water, usually with a strong earthy odour.
Seafood caught in impacted areas may also be at risk.
‘‘People should not undertake recreational activities where they may be coming into direct contact with the water such as swimming or water skiing, as well as domestic uses including showering and washing,’’ the statement said.
‘‘Common sense would suggest not to consume fish caught in water affected by a bloom.’’
Hunter New England Health has advised anyone potentially exposed to seek medical treatment.
Algal Information Hotline
1800 999 457