Health officials have warned exposure to the toxic algae bloom which has closed popular swimming beaches on Lake Taupo could result in breathing difficulties.
A public health officer told a media briefing this afternoon contact with the phormidium algae could cause eye irritation, breathing difficulties if inhaled, and stomach upsets.
"It also produces a neurotoxin, which causes tingling, numbness and difficulties with breathing," he said.
A week of hot, settled conditions has meant Taupo residents and visitors have been flocking to Lake Taupo to cool off.
After the algal blooms were discovered to be forming in mats on the lake floor near shorelines, samples were taken at five sites earlier this week and the test results came back yesterday.
The warning for people to avoid swimming in affected areas - which encompass almost all of Taupo's popular swimming beaches - has been greeted with widespread dismay on social media.
Said one mother: "Should I be worried as my son was in lake most of the weekend just been and has had asthma since?"
Taupo's main lakefront, Acacia Bay, Five Mile Bay, Kinloch and Whakaipo Bay are all included in the affected sites where people are advised not to paddle, wade, swim, or do anything that might mean they come into direct contact with the algal mats or swallow lake water.
While the algae is naturally-occurring and blooms are triggered by warm water temperatures and hot settled conditions, it also feeds off nitrogen and phosphorus in the water.
Twelve years ago, the-then Commissioner for the Environment Dr Morgan Williams warned that the worst effects of pollution were yet to be seen on the Central North Island's lakes.
Research had shown it took many years for nutrient-enriched groundwater to move through the region's large volcanic aquifers, so the effect of intensive land-use had yet to reach the lakes.
That would see high, bloom-causing nitrogen concentrations increase significantly in the future.
But progress, especially around Lake Taupo, has been made since his warning.
The Lake Taupo Protection Trust was set up in 2008 with the aim of reducing by 20 per cent the amount of nitrogen getting into Lake Taupo from manageable sources, such as leaching from farms and discharges from wastewater plants.
It had a $81.5 million public fund and reached its target of reducing nitrogen entering the lake by 170 tonnes in 2014, three years ahead of schedule.
The project successfully encouraged land owners to change their farming practices and significantly reduced the amount of nitrogen getting into the lake, by buying some farms, paying farmers to convert from farming to forestry, and capping nitrogen leaching limits.
But stormwater still discharges directly into Lake Taupo and some of the lake shore settlement areas lack wastewater schemes. Taupo township has had three separate wastewater spills into the lake this year alone after wastewater pipes became blocked by people disposing of wipes and fat into the wastewater system.
In 2003 there was a toxic blue-green algal bloom (Anabaena circinalis) in Lake Taupo, which led to a swimming ban in parts of the lake. At that point it was only the second time an algae bloom was reported in the lake.
Cyanobacteria was also detected in Lake Taupo last year but levels remained low to moderate and were not of concern.
Taupo District Council this morning shut down its water supply at the lakeside settlement of Hatepe as a precautionary measure following the confirmation of the potentially toxic algae.
Operational services group manager Kevin Strongman said the intake for the Hatepe supply was much shallower than the intakes for other supplies and it was important the council erred on the side of caution until test results were available on Monday.
The council would truck in water for the 100 or so properties affected until the water testing results were known.
Other water supplies around the Taupo district were so far unaffected.