A bloom of potentially toxic blue-green algae has prompted a health warning for Lake Okaro, near Rotorua.

The health warning comes after routine monitoring results provided by the Bay of Plenty Regional Council confirmed a bloom of potentially toxic blue-green algae.

"The health warning means that people should avoid any activity which results in significant contact with the lake water," Toi Te Ora medical officer of health Dr Neil de Wet said.

Contact with algal blooms can cause asthma and hayfever attacks in susceptible individuals, as well as skin rashes, stomach upsets, and in some cases neurological effects such as tingling around the mouth, headaches, breathing difficulties and visual problems.

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Signage will be erected at Lake Okaro advising potential lake users about the algal bloom.

Monitoring will continue and any changes in advice will be communicated accordingly.

In February this year, Toi Te Ora lifted a health warning from November 2018, after algae blooms had subsided in the lake.

At the time medical officer of health Dr Phil Shoemack said algal blooms could develop rapidly and unpredictably.

"Experience shows that Lake Okaro is prone to algal blooms at any time of the year.

"Lake users must exercise caution and make their own judgement about water quality by looking at the colour and clarity of the water, and whether it smells at all."

The public can obtain up-to-date information on this health warning and others for the Bay of Plenty and Lakes districts by checking the Toi Te Ora website and social channels.