Mr Kelly said this Saturday's event had already sold out.
He said costumes were a massive part of the race, with prizes awarded to the best dressed. "It's become the Wellington Sevens of running events."
Mr Kelly said the event was as social or competitive as entrants wanted to make it.
"At some points you'll be up to your knees in mud and it's pretty sticky. Having to pull your limbs out is quite taxing."
Organiser Murray Fleming said the majority of competitors in the Rotorua event were from out of town.
"Rotorua is a magnificent destination to travel to in the weekend. People tend to come from all over the country."
Mr Fleming has been involved in organising the event since the first challenge 10 years ago, which had 420 entries. He said different venues were used and more obstacles were added each year to keep it fresh.
He said the event was very popular. "The atmosphere is just so positive and so different to anything else in the country. So people come back and they bring a lot of their friends."
The total number of competitors nationwide is expected to reach 11,000, with more than half of these in Rotorua.
Events have already taken place in Wellington, Auckland and Palmerston North.
Mr Kelly said the competition had so far been blessed with good weather.
"It's cleared up in the days before, which makes for perfect Tough Guy conditions. But if it's raining it'll be just as fun."
Burgen Tough Guy and Gal Challenge
When: Saturdays August 4, 11 and 18, 8.30am to 2pm
Where: Lakes Ranch outdoor pursuit and equestrian centre, 15 minutes east of Rotorua
After party: The Kingsgate Hotel from 5.30pm
Entries: $60 per person or $50 per person for bulk entries of 20 or more people