"The chances of safe rescue from either Lake Okataina or Lake Tarawera was minimal. [And] we did not [want] any runners entering a forest (due to tree danger) after 6pm since the danger of either tree or branch fall was significantly increased."
Both races will still start at the Redwoods Visitor Centre, on Long Mile Rd, at 6.30am.
The finish line for both races will now be at Boyes Beach at Lake Okareka.
Runners will make their way to Okataina Lodge before doubling back to Lake Okareka.
The Tarawera Ultramarathon Facebook page was swarming with comments last night, after the announcement was made.
Most of the people commenting agreed with the decision. One person wrote it was better to be safe than sorry.
"Tough call but rather have everyone safe and back next year than a tragedy. Good call."
Meanwhile, New Zealand's top four ultramarathon runners are still set to compete against each other today for the first time.
Christchurch's Vajin Armstrong will race alongside fellow Kiwi contenders Michael Aish, Scott Hawker and Martin Lukes during the Vibram Tarawera Ultramarathon.
Armstrong said the four Kiwis had never raced together at an ultramarathon before, so it was going to mark a special occasion in Rotorua.
"It is a bit of a Kiwi showdown," Armstrong said.
He said it would be very satisfying to cross the line ahead of the others, during the friendly rivalry.
The 33-year-old has finished on the podium at the Tarawera Ultra on three occasions and said he wanted to repeat that success again this year.
"If I can get back on the podium again that would be a big goal for me."
To do that, Armstrong will have to get past some of the best long-distance runners in the world.
The big names include defending champion Sage Canaday (USA), Michael Wardain (USA), and Brendan Davies (Australia).
Defending women's champion Kiwi Ruby Muir has pulled out of the race meaning American Meghan Arbogast will be favourite to win the 100km women's race today.
Today's event will include 850 participants.
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