Police have spoken to both men, and it is understood each blames the other for starting the fight. With no CCTV cameras in the carpark, officers have appealed to witnesses to help them with vital information.
The injured man was found bleeding and confused in the carpark area by a worker at the events centre.
"They actually found the guy after the event. He was conscious. He'd been hit in the face," a source said.
"He was there on his own and the other guy was gone."
The victim's car was parked around the side of the building, away from the main front doors, the source said.
"The good thing is it wasn't in front of the kids while they were playing. It's very strange."
It wasn't clear whether children were present with either of the men at the time.
There was some suggestion one of the men had cut the other off while driving to the match, and the animosity continued during the 8.30pm Intermediate League 1 game and had spilled over afterwards, the source told the Herald.
Police say one of the men was driving a red Holden Commodore and the other a silver 4WD Toyota Highlander.
It's not the first incident of a fight at a children's school event between adults in recent years.
An under-11 rugby game in Martinborough, in Wairarapa, in July had to be abandoned after a sideline brawl involving parents.
The fighting was sparked by a disagreement about something that had happened on the field.
"People have got carried away," Wairarapa Bush Rugby Union chairman Paora Ammunson said at the time.
"There was violence and a confrontation. This was parents behaving badly."
Last June in Auckland, a Metro Football Club coach and an Eastern Suburbs supporter were disciplined after a fight during a 13th-grade girls match.
The coach was issued a four-match suspension and stood down from the club, while the supporter was banned for one match and suspended from watching his daughter play for the remainder of the season and the next.
Can you help? Anyone who witnessed the fight or has information that could help police should contact Constable David Cooper on (09) 477-5000.