Lake Rotoroa (Hamilton Lake) has had a health warning issued this morning, after a sudden algal bloom was recognised there yesterday.

Waikato District Health Board's Population Health service endorses the information released by Hamilton City Council earlier today, urging people and their pets to stay well clear of the lake's waters until the bloom subsides.

"Although previous monitoring of this lake typically has shown very low cell counts, the possibility that blooms could occur has always been recognised, even though the surrounding land is entirely urban," said Waikato DHB medical officer of health Dell Hood.

"The popularity of this lake with Hamiltonians means that health risks must be taken very seriously, particularly for young children, whose small size makes them more susceptible."

The most likely symptoms include rash, skin and eye irritation, allergy symptoms such as hay fever and asthma and possibly stomach upsets such as diarrhoea and vomiting.

These effects may not appear until some time after contact.

Population Health would like to be informed about health problems which develop after exposure to any of the Waikato lakes, even if they have recovered fully.

Phone (07) 839 8899 in and out of hours for public health advice.