High concentrations of toxic blue-green algae have forced Masterton District Council to ban all recreational pursuits on Henley Lake.
Parks and facilities manager Grant Hathaway yesterday issued a health warning, saying water in the lake was unsafe due to the algae, which is also known as cyanobacteria, and the lake would be closed until the warning was removed.
Blue-green algae produces toxins that are harmful to both humans and animals if the water is ingested or comes in contact with skin.
Symptoms of illness include skin rashes, nausea, upset stomach, and tingling or numbness around the mouth and the tips of fingers.
Dogs are especially at risk and outbreaks of algae have been blamed for several dog deaths throughout the country.
People walking dogs around Henley Lake have been asked to make sure their animals are kept away from the water and the lake's edge.
The district council monitors the algae weekly at Henley Lake during summer.
Toxic algae blooms have affected Henley Lake in past years and also the Waipoua River.
It forced the cancellation of a dragon boat regatta in 2008.
Blue-green algae are microscopic organisms that are naturally present in many waterways, including relatively "clean" water.
A warning has also been issued for toxic algae in the Waipoua River in Masterton this week.