People are urged to avoid any contact with Lake Ōkaro after recent tests confirmed algal bloom activity.
Toi Te Ora Public Health Medical Officer of Health Dr Phil Shoemack said the health warnings follow a decision last November to place a warning on Lake Rotoehu for the same reason.
People should avoid any contact with water at both lakes, he said.
Contact with water affected by blue-green algal blooms can cause asthma and hayfever attacks in some individuals.