There's a stark reminder today of why warmer water temperatures are not always welcome.
Just a day after the water of Lake Taupo measured a record 26degC, health authorities have warned people they shouldn't swim or even go up to their ankles in it, because of algal bloom.
It means organisers of this weekend's Ironman have had to cancel the swim leg of the course and the hot conditions mean the algae is unlikely to disappear anytime soon.
There's one obvious reason Lake Taupo is a tourist drawcard, but the water-loving visitors who flocked here to cool off on Friday were warned any contact with the water could make them sick.
Scientists have found an algal species never seen here before - it's growing like a mat on the floor of the lake and floating up in brown clumps.
"Contact with this species can cause health effects - skin rashes, eye irritation, breathing difficulties if you inhale some of the material," says Toi Te Ora Public Health service medical officer Dr Neil de Wet.
In more severe cases, it can cause an upset stomach, muscle tingling and weakness. Dogs are especially vulnerable.
The potentially toxic algal species has been detected in all five tests taken across the lake so far - at Five Mile Bay, Acacia Bay, the Town Lake front, Whakaipo Bay, and Kinloch Beach.
It's likely to persist over summer. It's flourishing because, over decades, nitrogen and phosphorous have leached into the lake from farms and wastewater plants.
However, the main trigger is the still, warm conditions and the high lake temperatures.
"The highest temperature we measured was 26 degrees," says water scientist Dr Eloise Brown. "I have not seen a temperature that high at this time of year in the lake. "Last summer, the highest temperature recorded was 23 degrees."
The algal bloom has forced organisers of this weekend's Ironman event to scrap the lake swim and made the course a three-kilometre run, followed by a 90km bike and another 21k run.
"We're aware that some people will be very disappointed that we've had to change the event for this reason, but the playing field is the same for everyone who is taking part and we hope everyone still has an amazing event," says Ironman regional director Chris Randle.
Some ignored or were oblivious to the warnings, but health authorities say, while it's ok to eat the lake's trout if it's washed thoroughly in fresh water, shellfish and crayfish are off limits until the tests for toxins come back early next week.