Duddings Lake pictured early last year with blue-green algae. Photo / File
The discovery of toxic blue-green algae at Duddings Lake in the Rangitīkei District has sparked a health warning.
Horizons Regional Council said its tests showed very high levels of the algae.
Health warnings would be placed at the lake, between Bulls and Turakina, and there would be regular monitoring.
People were advised to avoid contact with the lake water until the warning was removed.
Blue–green algae might produce toxins that are harmful to humans and animals if swallowed or through contact with skin, such as may occur when swimming, water skiing or kayaking.
Exposure to toxins could cause skin rashes, nausea, tummy upset and tingling and numbness around the mouth or fingertips.
Anyone who experienced symptoms after contact with contaminated water was advised to visit a doctor immediately and let them know about the contact with the lake water.
Blue green algae occurs naturally but can increase rapidly during summer, known as a "bloom".
If the water was cloudy, discoloured, or had small globules suspended in it, people were advised to avoid all contact.
Boiling water would not remove toxins and drinking the water should be avoided at all times.
Fish could concentrate these toxins and consumption should be avoided.
If fish were eaten, the gut and liver should be removed and they should be washed in clean water.
The public can access information on and find out if any health warnings or cautions are in place by visiting www.lawa.org.nz/.
Police said those involved were known to each other.