Media release

Cyanobacteria confirmed in Lake Hayes

Monday 17 February 2020

People are advised to keep out of the lake and keep dogs well away for the time being.

Testing today confirmed that algae in a water sample collected from Lake Hayes is the potentially toxic cyanobacteria, blue-green algae (Anabaena lemmermannii).

Cyanobacteria has the potential to produce toxins that are harmful to humans and pets if swallowed or through contact with skin.

Warning signs were put up around the Lake Hayes recreational reserve on Saturday evening due to the green-coloured algae in the water.

Although the discolouration of the water is currently quite localised, possibly due to wind concentrating the algae in one area, toxins from cyanobacteria may be present throughout the whole lake. That means it is safest to stay out of the water, even if it looks clear.

Another water sample will be taken for testing today as part of weekly cyanobacteria monitoring. The results of that sample (due on Wednesday) will indicate the ‘biovolume’ of cyanobacteria, which determines if the lake is safe for swimming.

This warning will remain in place until weekly testing shows a reduced biovolume of cyanobacteria to safe levels.


Falls Dam

ORC staff have also found a suspected cyanobacteria algal bloom in Falls Dam this afternoon, and have taken a sample to determine if it is potentially toxic. Staff will undertake further sampling in the Manuherekia River tomorrow to determine the concentration of algae downstream of the dam.

In the meantime, as with Lake Hayes, people are advised to stay out of the water and keep pets away. ORC will provide an update on sampling from Falls Dam and the Manuherekia River in the coming days.


What to do

Avoid contact with Lake Hayes and Falls Dam, even parts that look clear, as toxins can persist after an algal bloom has dissipated or moved.

Anyone who suspects they are experiencing a reaction due to contact with toxic algae should seek medical attention.

If you spot green algae in lakes, or dark mats in streams and rivers, contact ORC on our 24/7 pollution hotline, 0800 800 033.

For more information on potentially toxic algae, see LAWA’s factsheet:


Cyanobacteria algal bloom in Lake Hayes on Saturday.

Cyanobacteria algal bloom in Lake Hayes on Saturday.

Green algal bloom photographed by ORC staff at Falls Dam this afternoon.

Green algal bloom photographed by ORC staff at Falls Dam this afternoon.