While one of the men was able to be helped away on his feet after contact, another slumped to the ground and appeared to have a seizure.
An event planned for Williams Park in Māngere yesterday did not go ahead and another planned for Marist Saints today was also scrapped.
The Run It Straight competition at Auckland's Trust Arena last night had drawn in more than 1000 people.
Eight men put their bodies and brains on the line for $20,000 prize money. Two required medical attention after being knocked out.
The winners from this week's trials will progress to a final competition held next month, where the champion is set to receive a prize of $250,000.
The sport has a number of high-profile endorsements, including Kiwi league legend Manu Vatuvei.
"This is one of the contact sports that I love, you know, it's a one-on-one… it just reminds me of training, we used to run at each other and try smash each other, just to get the feel of the contact," he told RNZ.
Others RNZ spoke to at the event said they liked the energy of the collision - "the big oomph", one said.
Another spectator said they enjoyed the vibes of the sport and the event.
"Just the big hits eh, all the hype it brings, the crowd, everything, the atmosphere, they said.
One fan told RNZ she was glad the sport was expanding.
"I'm glad that this one went through, and now the world can see what type of sport this is, and you know the people who are attracted to it.
"It's a good thing for our brothers, but they need to do a woman's one, I'm gonna run it straight," she said.
The sport has been under fire for its risk of head injuries.
Wednesday night's event saw knock outs - including one athlete appearing to have a seizure as he hit the ground.
When RNZ approached some fans about the criticisms of the sport, some still backed it.
"Mind your own business eh, like you know, just let the people be, let them be," one person said.
Another commented "the people wanna do it, like why stop them?".
Vatuvai said he backed the sport despite the controversy.
"I'm just happy that it's here in New Zealand, I know a lot of people hate it and a lot of people love it, I'm one of those people that love it."
When asked whether he would be tempted to try the sport himself, he said: "I do, I do, honestly I do, but it's one of those things that my mind is telling me one thing but my body is telling me another".
Auckland Council said the event planned for Williams Park in Māngere was cancelled after organisers failed to secure a permit.
The council said the organisers did not seek a permit or try to book council sites, but if they had followed the correct process, it was highly unlikely the event would have met its guidelines for approval.
Earlier, RNZ spoke to a brain injury support group who said they were "appalled" that Run It Straight events were being staged across Auckland.
Chief executive of the Headway charity Stacey Mowbray said the activity was undoubtedly causing brain injury.
The format and promotion of the events appeared to exploit financially vulnerable people by offering large cash prizes, she said.
