According to WaterNSW, a total of 20 NSW rivers and dams are under the red alert (high alert) for blue-green algae.

A WaterNSW spokesperson said blue-green algae is a naturally occurring phenomenon and its outbreaks occur throughout the warmer months across the state. 

"Nutrient levels in water, warmer weather, sunlight low flows and lack of replenishment inflows into storage are among the contributing factors," the spokesperson said.

WaterNSW is expecting the prevalence of algae to decline with the cooling weather, but warned residents to avoid recreational activities where they may come into direct contact with the water, such as swimming, as well as domestic uses including showering and washing.

"Contact with the water may also pose a threat to livestock and pets. Anyone worried they may be suffering ill-effects from contact with blue-green algae where a red alert is in place should seek medical advice," the spokesperson said.

"Boiling the water does not remove algae toxins." 

Experts believe it is a bit unusual, but not uncommon.

"I think one of the main reasons [for blue-green algae in the Central West dams] is that water is getting hotter and there is less inflow of water coming in to refresh the existing water in the dams," Orange and Region Water Security Alliance co-ordinator Cyril Smith said.

"At the moment, I don't think we can do much about it. It is going to take good rainfall to get some inflow into the dams to refresh the water. Cooler temperature will also help in coming months."

The Central West is going through one of the worst droughts in 100 years and recent heatwaves have worsened conditions in the region.

Most dams are operating below their storage capacity.

Chifley Dam was operating at 47.9 per cent capacity on March 12 and Gosling Creek and Windamere dams were at 55.37 per cent and 34 per cent capacity.