Mafi faced a Dunedin court on Monday morning after being charged by New Zealand police over the incident, and was released on bail.
Both players were fined $16,300 (AU$15,000) by the club and are unlikely to play for the Rebels again.
On Tuesday, Melbourne Rebels CEO Baden Stephenson said: "Notwithstanding that both Mafi and Timani were finishing at the Rebels this season and heading overseas to continue their playing careers, it is only appropriate, given the seriousness of the incident, to sanction both players with a significant fine.
"The actions of these two players is totally contrary to our Club's values and has tarnished the excellent work of so many in the club, both on and off the field this season."