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.
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It can also cause skin rashes, stomach upsets, and in some cases neurological effects such as tingling around the mouth, headaches, breathing difficulties and visual problems.
Shoemack said while there have not been any recent reports of people becoming ill from contact with algae, or their toxins from Lake Rotoehu or Ōkaro, it's important to stay vigilant and out of the water.
Rotorua Lakes Council will be place signs at Lake Ōkaro advising potential lake users of the bloom and encouraging people to stay up to date with health warnings for the Bay of Plenty and Lakes districts.