Health officials are asking people to stop recreational activity on Lake Nga Roto, north of Te Awamutu, due to a significant concentration of blue-green algae

The National Public Health Service has issued a warning against swallowing or skin contact with affected water.

It says the toxins can trigger asthma, hayfever, rashes, stomach aches, headaches, breathing difficulties, visual problems, and tingling in the mouth.

Anyone who comes into contact with the water is advised to shower and change their clothes as soon as possible.

Warnings are already in place for Hamilton Lake, as well as others around Waikato lakes Hakanoa, Waikare, Rotokauri, Whangape and Kainui.