The alert, issued by the Hunter Regional Algal Coordinating Committee comes after recent sampling revealed algae numbers in the high range.

Hunter New England Area warned people not to undertake recreational activities that will lead to contact with water in the affected areas.

This includes swimming, water skiing, showering and washing in the water.

Gordon Grainger of the Myall River Action Group said while he understood blue green algae was a natural occurrence in the Myall Lakes he also understood it decomposed when it met normal seawater.

"However for some time now, we have experienced very low salinity levels in the Lower Myall River and the algae is present past Tea Gardens," he said.

The MRAG has been campaigning for over three years to have the natural entrance dredged and cleared of sand blockage.

"The river has suffered the loss of the oyster industry, outbreaks of fish disease, poor water clarity and now this outbreak which will have a further adverse impact on the community," he said.

A spokesman for Hunter New England Health said people should be mindful that algal blooms can be blown around the lake.

Blue-green algae appears as green paint-like scums on the water and foreshore edges or clumps throughout the water and usually has a strong earthy odour.

Coming into contact with it may cause severe stomach upsets, nausea and skin irritation in people.

Hunter New England Health has advised people not to eat mussels, crayfish or the internal organs of fish from high level blue-green algae alert areas.

Fish caught in water affected by a bloom should be washed thoroughly in uncontaminated water and any internal organs disposed of before consumption.

Warning signs are being erected in the affected areas and information updates about blue-green algae blooms and red level warning areas can be obtained from the Regional Algal Coordinating Committee's freecall algal information hotline on 1800 999 457 or at www.water.nsw.gov.au.