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Published: 2006-03-02 00:00:00

A health warning has been issued for the Waikato River from Lake Karapiro to the sea, after elevated levels of potentially toxic blue-green algae were found in the water.

Water samples collected at three regularly-monitored sites along the Waikato River on Monday revealed high counts of blue-green algal cells belonging to the genus Pseudanabaena.

Counts (measured in cells per millilitre of water) were:

  • Lake Karapiro – 15,300
  • Waikato River at Ngaruawahia – 22,300
  • Waikato River at Te Kauwhata (at the point where Waikato District Council abstracts drinking water for the town) – 22,500.


The recreational guideline for blue-green algae is 15,000 cells per millilitre of water.

“Algae are always present in the river and most of the time the types present are harmless,” said Environment Waikato Water Scientist Bill Vant.

“However, from time to time, numbers of potentially harmful algae increase. Blue-green algae can produce toxins that create water-use issues.”

Environment Waikato is working with a number of other agencies to monitor cell counts and toxin levels at nine sites along the Waikato River from Taupo to Tuakau.

“The current information we have is that toxin levels are low – but high cell counts mean there’s a risk toxin levels could increase,” Mr Vant said.

“We will be keeping a close eye on the situation and stepping up our monitoring operation as necessary.”

Information on cell counts in the Waikato River are available on Environment Waikato’s website, with updates provided as they come to hand.

Mr Vant said there was potential for algal counts to remain high or increase if the weather stayed fine and calm.

“If we get very unsettled weather in the next few days, the bloom could collapse and the counts could fall.”

Waikato Medical Officer of Health Dr Dell Hood is warning people not to swim in, or carry out any other recreational activities such as kayaking or water skiing, on the river.

A health warning for the Waikato River was last issued due to high blue-green algal levels in the summer of 2002/2003, when counts of Anabaena planktonica rose to 35,000 per millilitre of water.

The latest cell counts at monitoring sites upstream of Lake Karapiro were below health guidelines.