Loose forward Mafi will appear in Dunedin District Court on Monday, charged with injuring with intent to injure in an incident in South Dunedin.
It's a dismal postscript to the Rebels' bold, but unsuccessful attempt to secure a Super Rugby finals berth, when they lost 43-37 to the Highlanders on Saturday.
While police didn't name Mafi, Rugby Australia had earlier confirmed the Tongan-born Japanese international loose forward was being held in police custody.
"The alleged victim, also a 28-year-old man, received moderate injuries following the incident," a police spokeswoman said.
"He did not require hospitalisation. The pair were known to each other.
"As the matter is now before the court, police are not in a position to comment further."
RA's statement on Sunday said it was aware of the incident.
It's understood no other players were involved.
"Both players remain in New Zealand, with Mafi currently in police custody, following an altercation between the two players," RA said.
"I am bitterly disappointed that an incident has occurred at the end of a season where we have taken pride in our on and off-field behaviour," said Melbourne Rebels chief executive Baden Stephenson.
"We will respect the process and won't be making further comments until all investigations have taken place."