WATERNSW has issued an updated warning for blue-green algae along the Murray River between Euston and Wentworth.

People are urged to avoid all contact with the water along the stretch of river, including at Mildura, Red Cliffs and Merbein.

On Wednesday, WaterNSW issued a red alert for potentially toxic blue-green algae in the river at Wentworth and Mount Dispersion, near Euston, but on Thursday the warning was extended to include the entire stretch of river between the two sites.

Current conditions indicate there is a high likelihood blue-green algae could be present at red alert levels along much of the river in this region.

While awaiting results from further monitoring in the area, WaterNSW advised the community to exercise a high degree of caution.

Residents and visitors are advised against entering the water, or drinking or bathing in untreated water, or recreational activities involving contact with water while the alert is in place.

Boiling the water would not remove algal toxins, WaterNSW said.

Any fish caught should be cleaned and washed thoroughly and any internal organs disposed of before consumption, and mussels and crayfish from red alert areas should not be eaten.

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

They make the water appear dirty, green or discoloured and generally give it a strong musty or earthy odour.

The alert is based on routine sampling and dense visual scum observed at Wentworth.

WaterNSW could not predict how long the algae would remain at high levels. 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 are advised to seek medical advice.

Updates about blue-green algae blooms and red level warning areas can be found at waternsw.com.au/water-quality/algae or by calling 1800 999 457.