WaterNSW issued a red alert (high alert) algae warning on Friday afternoon, saying contact with the water could also pose a threat to livestock.

Bathurst Regional Council has closed the dam to all recreational activities, with the algae likely to cause gastroenteritis in humans if consumed, and skin and eye irritations after contact.

The high alert algae warning comes just a week after the temporary Bathurst Aqua Park at the dam closed a month earlier than planned, citing low water levels and poor water quality, and comes in the same week as the water level at the dam has dropped below 50 per cent for the first time in more than a decade.

A spokesperson for WaterNSW said the algae outbreak should not impact on the Campbells River or Macquarie River, but warned people wo keep an eye out for algal scums.

"The storage offtake is located below the surface to reduce the algae entering Campbells River below the dam. Algae sampling in Campbells River below Chifley Dam has commenced," the spokesperson said.

"Algae results from the Macquarie River at the Bathurst Weir indicates that algae levels are at a No-Alert status, however, people should apply a level of caution and be on the lookout for any algae scums.

MAP: Where is Ben Chifley Dam …

"If scums are present, people should not enter the water or let their pets drink form the water. Stock and domestic users should also be on the lookout for algae scums and seek alternate water supplies if algae scums do become visible."

WaterNSW advises people not to enter the water, drink untreated water or bathe in water drawn directly from the dam while the red alert level warning remains in place. Boiling the water does not remove algal toxins.

Town water supplies remain unaffected and safe to drink.

"Livestock owners are reminded to continue to check stock water supplies for blue-green algae and to remove stock from foreshores where surface scum is visible or blue-green algae are suspected," the spokesman said.

"Blue-green algae usually appear as green paint-like scums on the water, near the edges, or as greenish clumps throughout the water. It makes the water appear dirty, green or discoloured and generally has a strong musty or earthy odour."

People should not eat crayfish from red alert warning areas and any fish caught should be cleaned and washed thoroughly in uncontaminated water and any internal organs disposed of before consumption.