A penalty count of 18-2 favoured Whakarewarewa during the match but it was Rotoiti who took their chances in the wet conditions.
Second half tries to Jesse Acton and Ben Swindlehurst put the game out of reach for the visitors and by the time the whistle blew for full-time Emery Park was in almost complete darkness.
Rotoiti coach Kevin Lee said his side was fortunate to win, while reclaiming the Geyser Land Challenge Shield, better known as the Tai Mitchell Shield.
"We were a bit lucky. The bounce of the ball went our way at times I thought," he said.
He said they had simply jumped on Whaka's mistakes to score their points.
Lee said it was nice to know they would be holding onto the shield until at least next year.
"If we lose every other game at least we will have won something," he said with a laugh.
He said to celebrate the win they were treated to some chops as part of the after match.
Whakarewarewa coach Maurice Stone said it was disappointing to lose the derby, with the long half time break hurting them.
"We were just getting on top at that stage and the long break didn't help us, especially our forwards. One or two started cramping up trying to warm up again," he said.
"But take nothing away from Rotoiti, they played well. When they got on top of us I felt the intensity in their team grow."
In other games, Tauranga remain unbeaten after knocking over second-placed Greerton 44-15, Mount Maunganui bet Te Puna 38-12, Te Puke Sports bet Paroa 34-5 and Rangataua drew with Opotiki 15-all.
Rotoiti 17 (Beau Keil, Jesse Acton, Ben Swindlehurst tries; Willie Ripia con) bet Whakarewarewa 3 (John Wiringi pen).
Division one results:
Rangiuru 31 Marist St Michael's 5, Ngongotaha 26 Ruatoki 15, Katikati 23 Whakatane Marist 15, Waikite 14 Kahukura 9, Te Teko 16 Poroporo 11.