Swallowing water and eating fish or shellfish from lakes affected by blooms should also be avoided
"If contact does occur, skin should be rinsed clean and clothing changed as soon as possible," she said.
"This warning is particularly important for children.
"If people still choose to use the lakes when warnings are in place, they should shower and change their clothing as soon as possible afterwards, even if no symptoms are noticeable."
Exposure to cyanobacterial blooms can cause rashes, skin and eye irritations and allergy symptoms such as hayfever and asthma.
It can also cause diarrhoea and vomiting and a tingling sensation and numbness around the mouth
The effects may be delayed for some time after contact with the affected water.
Dr Hood said there had been reports of stock dying after drinking the toxic water as well as several dogs that ate the algal bloom in other parts of the country.
"If 650kg of dairy cow can succumb to this you can imagine what it could do to a toddler."
The blooms, which are caused by nutrients such as nitrogen and phosphorus entering the water system, thrive in warm water temperatures and little or no wind.
They can be seen by the naked eye as discoloured water.
Waikato Regional Council is no longer routinely testing Lake Kainui but caution is always advised for users of this lake, because of its history of cyanobacterial blooms.
Dr Hood said the Population Health service would like people who developed health problems after exposure to any of the Waikato lakes to contact it immediately.
Danger alert
Cyanobacterial bloom
* Lakes affected: Hakanoa, Waikare, Waahi, Whangape.
* Exposure to the bloom can cause: rashes, skin and eye irritations, allergies, diarrhoea and vomiting.
