The swimming leg of the 70.3 Taupo half Ironman has been scrapped due to safety concerns over dense concentrations of algae in the town's famous lake.
A host of leading international triathletes have already registered for Saturday's race at Lake Taupo, which bills itself as the home of New Zealand ironman.
But after a Waikato Regional Council warning not to swim in the lake's water, they will now complete a modified course consisting of a 3km run, 90km bike ride and 21km run.
Race organisers say they were told at 8pm on Thursday that potentially toxic algal species, including phormidium, were present.
Testing overnight by NIWA scientists also confirmed the algae's presence in Five Mile Bay, Acacia Bay, Whakaipo Bay and Kinloch Beach.
Race director Wayne Reardon said organisers immediately rushed to alert competitors to the changes and what options were open to them.
"We know athletes will be disappointed," he said.
"However, our team are still committed to putting on a fantastic race on Saturday, albeit in a modified format."
Disappointed competitors will also have the opportunity to pull out of the event and transfer their entry at no cost to any 2018 Ironman 70.3 race in Australia or New Zealand.