Several Taupō lakeside settlements have had their water intakes shut down over fears of a potentially toxic algal bloom.
A public health warning has been issued urging people not to swim at sites including the main lake front, Five Mile Bay, Acacia Bay, Whakaipo Bay and Kinloch Beach.
Ten days of hot, dry, and still conditions are being blamed for the algal blooms.
At peak summer season it means big interruptions for tourism and sporting activities on the lake.
The commodore of Lake Taupō Yacht Club, Alan Simpson, said today they had to suspend their busy summer program for all centre-board sailing.
"It is all recreational sailing so it's our learn-to-sail programmes, it's our have-a-go programme which were the two events we had on the lake today.
"We've got organised coaching and also centreboard racing as well. So none of that will happen until the bloom goes unfortunately.
"We've just got to manage it, it's just one of those things you have to live with unfortunately.
Some tourism operators said they wanted certainty about the danger before disrupting their water activities while others had already cancelled some activities on the lake.
The swim leg of tomorrow's Ironman competition has been replaced with a run after yesterday's testing.
Waikato Regional Council water quality scientist Eloise Ryan said they needed to know more about the algae so tests were being taken to a lab in Nelson.
"One of the species is called Phormidium and it's a known toxin producer.
"Now, I'm not saying it's producing toxins right now, we don't know that, so today I'm collecting samples right now and I'll have those results back by Monday.
"We've got scientists all over the country working this weekend to test those water samples."
The algae can cause skin rashes and an upset stomach and, if swallowed, can cause weakness, numbness and breathing problems.
She said she had never seen so much of this type of bloom in the lake in 15 years of studying it.
"I have to say I've never seen such record temperatures in the bays here.
"I was out sampling here on Wednesday and measured a water temperature of 26°C. Now the peak high I saw last year was 23°C, so we haven't had such high temperatures this early in the year.
"With these unexpected high temperatures it seems there's a species of algae that can take advantage of these hot still conditions."
Water cut to properties
This afternoon public warnings were expanded, with people on private water supplies that used water from the lake urged to turn them off.
Taupō District Council has shut down its intake at Hatepe after drones spotted what could be algae near the pipe inlet.
The intake at Motuoapa has also been closed over concerns around the water's clarity.
Mayor David Trewavas said the risk was low and water tankers from nearby Turangi were helping to supply the area's 600 properties.
"We've taken a precautionary approach to close off the water.
"We've got staff now going door to door, putting notices out so people are well advised of the situation."
The council said there was currently no risk to other water supplies and it had upped its testing at its other intakes.
The mayor said temperatures needed to cool for the algae to go away.
"I thought I'd never say this as mayor but the lake needs to be rough and it needs trumped up and we need some rain and some wind. We need a storm to come through just to disperse it all."