After the game, the two players, who grew up together in Tonga, went to a South Dunedin home where they let loose among Mafi's family members.
By 4am the next day, the men were intoxicated.
Mafi objected to what he believed was inappropriate language being used by Timani in front of his female relatives, the court heard.
He challenged him to a fight.
The judge called it "an extreme overreaction".
The burly forwards exchanged blows in the lounge but were eventually separated by others in attendance.
Fearing for his safety, Timani fled the house.
Mafi, however, found the victim in Bathgate Park, where the violence continued.
The defendant punched him repeatedly in the head and Timani believed he was also kicked numerous times.
Mafi said he stopped punching when he believed Timani had "had enough".
A police summary revealed the victim was "escorted" to a waiting vehicle but managed to flee when they stopped at traffic lights.
The defendant admitted inflicting the blows and said he had become "enraged" by his teammate's coarse language.
In the days after the assault, the victim was pictured with a heavily swollen face and two black eyes.
Timani – who is currently contracted to La Rochelle, in France - told the Sydney Morning Herald he thought he was going to die during the sustained beating.
Both men were fined A$15,000 for breaching team protocol.
In a victim impact statement, Timani said the ordeal had had a profound impact on him, both financially and emotionally, which was ongoing.
He said he believed the assault – and the "concussive symptoms" stemming from it - could have shorted his playing career by a year.
Judge Macdonald said documents showed there was an appreciable risk Mafi would lose his Japanese rugby contract if convicted.
The value of the contract, while not specified in court, was probably "significant if not substantial", he said.
While having "some reservations" the judge said he was prepared to accept that outcome was out of proportion to the gravity of the offending.
Mafi, who has no previous convictions, must pay Timani the $50,000 reparation within a week, Judge Macdonald ordered.
A letter of apology would also be passed on to the victim, the court heard.