A HIGH alert level blue-green algae warning has been implemented for a 20km section of the Murray River between Psyche Bend and Merbein, as well as Kings Billabong.

The declaration of a red alert follows testing undertaken by Lower Murray Water at a number of sites and surface scums have been observed at a recreational location downstream from Mildura.

Signs are being erected along the affected reach of river and the red alert is expected to be in place for at least two weeks, depending on ongoing monitoring results.

WaterNSW said it was also monitoring water quality at locations upstream, downstream and within the red- alert area and would report the results as soon as they became available.

Levels of blue-green algae continue to fluctuate in the lower reach of the Murray River.

A red alert indicates that people should not undertake recreational activities where they may come 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 and people should not eat fish, mussels or crayfish from red alert warning areas.

Blue-green algae usually appear as green paint-like scums on the water, near the edges, or as greenish clumps throughout the water.

Blue-green algae occur naturally and can reproduce quickly in favourable conditions where there is still or slow-flowing water, abundant sunlight and sufficient levels of nutrients.

Regular monitoring will continue and the alert will be lifted as soon as the high levels of algae dissipate.

People who believe they may have been affected by blue-green algae should seek medical advice.