About six Sharks players were becoming aggressive because one of them did not have identification and was being denied entry to bars, he said.
"One of the players got really aggressive to door staff ... he came back and apologised to him, and started to rehash the argument. And then king-hit him across the face, totally unprovoked.
"When that happened, a couple of the other guys came running in and started getting into the bouncer who had collapsed to the ground, kicking him while he was down.
"We had another bouncer get knocked out. When I turned to help, one of them came at me and punched me right in the sternum. He then backed up and started kicking a bouncer, who was on the ground, in the head. They're big, tall guys, using their reach, running in using jabs to the head, and then running back out."
Mr O'Leary said he had no knowledge of bar staff returning punches.
"We were trying to defend ourselves, or restrain, waiting for police to arrive."
The players then ran off in two groups, and were picked up later by police, he said.
Mr O'Leary said he went to hospital to have his chest checked out, and was cleared to go home. Others were checked at the bar, and did not have serious injuries. The man who was allegedly knocked out regained consciousness quickly, he said.
"He was pretty groggy, but he is fine today. There are few sore bodies and heads around today, that's for sure. It could have been a lot worse."
He said the players seemed "pretty sober" because they could not gain entry to any bars to celebrate their last-second win over the Taranaki Mountain Airs.
Video footage of the attack had been handed to police, he said.
National Basketball League chairman Sam Rossiter-Stead he was taking the incident "very seriously".
"I have spoken to the Southland Shark management. To say they are disappointed would not be accurate. They're devastated by what's allegedly happened, as indeed we are.
"Players are role-models to our community, whether they like it or not. Anything that in any way that brings the competition into disrepute will be dealt with very harshly."
He said the NBL had a "zero-tolerance" stance towards violence.
"If these allegations are proven, we will be taking the strongest possible action under our own processes. The NBL has never been in a healthier state. There are too many people working too hard to make a success of this competition to allow a small minority's actions to diminish that."
