Health authorities have warned people to stay clear of Lake Karapiro and a popular 150km stretch of the Waikato River that may have harmful levels of toxic algae.
The Waikato District Health Board said yesterday all water users should avoid contact with the lake and stretch of river from the
lake to the sea.
The threat could last for weeks.
Routine testing this week showed high counts of potentially toxic blue-green algae belonging to the genus Pseudanabaena. The toxins can cause asthma attacks, vomiting and diarrhoea, skin rashes, and irritation of eyes, nose and throat.
"Right now we know that toxin levels in the river are low, but because we've got high algae levels, there's always a risk that toxin levels will increase," said Bill Vant, water scientist for Environment Waikato.
The recreational guideline for blue-green algae is 15,000 cells for every millilitre of water; the last tests showed between 15,300 to 22,500 cells at three regularly monitored sites.
"The guideline is based on the cell counts, because you're not always in a position to check whether there are toxins associated with those cells."
Mr Vant said people had died from contact with toxic algae but only under extreme circumstances.
Waikato medical officer of health Dell Hood said this was the first time this species of algae had reached such high levels, and little was known about its health effects.
There was potentially a higher risk of skin rashes because this type of algae typically formed long strands that could become trapped in clothing, she said.
"People who choose to use the river against health advice must avoid swallowing any water and should change wet clothing and shower as soon as possible after contact with the water."
Despite the warning, Watercare spokesman Mark McLauchlan said the company would not be shutting its Waikato treatment plant, which draws river water to be mixed with water from the Hunua dams, then distributed throughout Auckland.
"The plant was built specifically for this sort of situation," Mr McLauchlan said. "The particular alga blooming at the moment is a little bit more aggressive, but it's nothing our plant can't handle."
Mr Vant said if the weather remained fine and calm, the bloom "could go on for weeks".
- Additional reporting: Errol Kiong
Health authorities have warned people to stay clear of Lake Karapiro and a popular 150km stretch of the Waikato River that may have harmful levels of toxic algae.
The Waikato District Health Board said yesterday all water users should avoid contact with the lake and stretch of river from the
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