A health warning has been issued for Ohau Channel, the channel that links Lake Rotoiti and Lake Rotorua.
Routine water test results provided by the Bay of Plenty Regional Council have confirmed the presence of blue-green algal blooms which are potentially toxic.
"The health warning means that people should avoid any activity which results in contact with the water in the channel," medical officer of health for Toi Te Ora Public Health Dr Phil Shoemack said.
Contact with water affected by blooms of blue-green algae can cause asthma and hayfever attacks in some individuals.
Contact with the blue-green algae 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.