A BLUE-green algae red alert warning now includes a 40km stretch of the Murray River below Merbein, including Wentworth and Fort Courage.

The red alert issued for the Psyche Bend to Merbein section of the river remains current.

The declaration of a red alert followed field observations undertaken by WaterNSW personnel revealed dense surface scums were now evident.

WaterNSW said increased water quality monitoring will be undertaken upstream and downstream from the red alert area.

The public were advised that levels of blue-green algae continue to fluctuate in the lower reach of the Murray River.

A red alert level warning indicates that people should not undertake recreational activities where they may be coming into direct contact with the water.

Contact with the water may also pose a threat to livestock, and pets.

The dominant cyanobacteria detected are known to be toxin producers and may produce a strong musty odour.

People are advised not to enter the water, not to drink untreated water or bathe in untreated water while a red alert level warning is in place.

Boiling the water will not remove algal toxins.

People should not eat fish, mussels or crayfish from red alert warning areas.

Updates about blue-green algae blooms and red level warning areas can be obtained by calling 1800 999 457 or visiting the WaterNSW website.