Evans quoted two racial slurs that MSP claims were audible on the sideline by OBU supporters.
He said the OBU president, captain and coaches were apologetic at the end of the game, but called for a joint investigation between Wellington Rugby Union and OBU.
"Sideline intervention [running on the field] by spectators and most critically racism must be approached head on before we start handing out sanctions to any players or clubs."
He said MSP was willing to participate and "assist in stamping out this blight on our game".
OBU chairman Pete McFarlane said the club was investigating the incident and "it would be unwise to speculate further" until it had all the information.
"MSP has made an allegation of a racist comment/s being made by a spectator/s in attendance at the conclusion of the game," McFarlane said.
"These allegations will be fully investigated."
They said OBU would not hesitate in taking action if the claims were found to be true.
"OBU does not condone or tolerate any conduct of this nature. To date, all parties who have been interviewed have denied any such conduct."
Wellington Rugby CEO Matthew Evans said there would be a hearing on Thursday evening to piece together Saturday's events.
While they had video footage of the incident, he said they did not have audio to verify the allegations of racist abuse.
In the coming days they hoped to have enough witness statements to corroborate what had taken place.
"We don't condone racism or any other form of abuse in any way, shape or form," he said.
"So if individuals are found guilty in that regard we will use the full extent of our powers to excuse them from our game. It's a zero tolerance sort of policy."
Wellington College has also been approached for comment.
Sports reporter Adam Julian was broadcasting the game from a pavilion and said the incident erupted after OBU scored a try that levelled the match.
"The next thing I witnessed was a Marist St Pats player leave the playing arena and make a beeline towards a spectator with the intent of assaulting that spectator and then everything was rather blurry.
"There were people who rushed in from the crowd and from the playing area, and there was shouting and shoving, and a highly inflamed and potentially dangerous situation, it was a miracle no one got seriously hurt."
He could not confirm the violence was sparked by a racial slur, but said there was definitely "a hostile atmosphere that seemed to be growing on the sideline".
The situation was not helped by spectators drinking on the sideline, which he said he could see from the pavilion.
"They've actually cracked down on the drinking and the crowd culture so this is actually highly unusual.
"This behaviour is actually the opposite of what typically happens and that's what makes it so infuriating."
Last November MSP was banned from a sevens tournament for three years after a brawl sent an opponent to hospital, and Julian said there had been instances of violence between the two clubs previously.
"It's the third instance of violence between these two clubs, so for whatever reason there seems to be a serious animosity between Old Boys University and Marist St Pats."
He agreed Wellington Rugby needed to seriously investigate Saturday's incident.
"What has become apparent and the previous two incidents involving OBU and MSP is that Wellington Rugby needs to take ownership of their own game and stamp these incidents out.
"These people who ruin it for others need to have the book thrown at them."
