ORC sampling of the thick scum on Friday confirmed the presence of cyanobacteria algae, which can produce toxins which may cause illness, such as skin reactions, in people and animals.

ORC director of engineering, hazards, and science Gavin Palmer said the warning signs at the lagoon outlet would remain in place until further notice because the scum was near public access points to Tomahawk beach.

“We have put up the signs because while the contaminated water is relatively shallow, it is still deep enough for children and dogs to swim in and potentially be at risk if they do so,” Dr Palmer said.

“We are urging parents to keep their children out of the water and dog owners to do the same with their pets.”

Dr Palmer said algae had been present earlier in the summer and then receded to very low levels. It was not possible to predict how long it would take to recede again.

ORC staff would continue to monitor the situation and update the public with developments.

If people see large amounts of green algae they should contact the ORC pollution hotline on 0800 800 033.