Event organiser Paul Charteris said the record for the 100km race was set at eight hours and 33 minutes and there was a group of "four or five guys" running this year who had the potential to beat that time.
He said most runners would complete the longest race in around 12 hours.
"It is officially going to be the most competitive ultra marathon in New Zealand history, we're breaking new grounds here in ultra craziness."
Mr Charteris said some of the top runners in the world would be running in the event and this was the first time such an event in New Zealand would be attracting so much international attention.
The difference between an ultra marathon and a regular marathon is primarily the distance, with a regular marathon set at a standard 42km.
However, Mr Charteris said this race was also difficult in many places.
"It is difficult around Tarawera and Okataina, but it is incredibly scenic."
There will be 13 aid stations set up along the 100km of track for runners to pick up food and drinks, as well as five points where a runner can leave a bag full of gear such as clothing, a change of socks, hats or a change of shoes.
Entries close today. To enter go to www.taraweraultra.co.nz.
Email: sonya.bateson@dailypost.co.nz