High levels of toxic algae have forced people to abandon their summer water activities at Henley Lake in Masterton.

Several groups of kayakers and people using the water yesterday were warned by patrolling council officers of the toxic algae at the lake which can cause skin rashes, nausea, stomach upset, and tingling or numbness around the mouth or tips of fingers.

Masterton District spokesman Sam Rossiter-Stead said recent tests of the water have shown the presence of cyanobacteria and it is recommended that visitors, particularly young children, pregnant women and the elderly, avoid contact with the water.

"The results came back last Friday and the levels had risen rapidly in the previous week, hence the move from green straight to red," Mr Rossiter-Stead said.

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"We updated the warning signage, our website and changed our advertising messages from 'don't forget to check the signs' to 'red alert'.

"Council officers undertake regular patrols of the lake to warn people about the dangers."

Dogs who have walked near the lake are also at risk from the toxic algae, and throughout New Zealand some dogs have been known to have died after eating cyanobacteria at the edges of water bodies.

"Keep dogs out of the water, and away from the edge," Mr Rossiter-Stead said.

"If you believe your dog has eaten contaminated material, contact your vet immediately."

Brochures with information regarding the toxic algae are available on the council's website and at their offices.