Bay batsman Alex Yates played a patient knock to post an unbeaten 53 off 120 balls to take his team to victory with four wickets in hand.
Jason Trembath was the pick of the visitors' bowling attack returning 2/24.
Batting first, Tauranga posted 204/9 against Western Bay of Plenty, which is made up of players from Mt Maunganui and Te Puke premier teams.
Tom MacRury top scored with a patient 68 off 130 balls and Sean Davey made 59. Their partnership of 89 defined the Tauranga innings.
Jeremy Bates took three wickets for 32, and wicketkeeper Tim Clarke took six catches.
Western Bay were in early trouble losing their top order relatively cheaply, before Ben Guild slowed the process with a handy 21 runs.
It was left to Josh Dwight and Dale Swan, who are better known for their bowling feats, to give the combined team a chance at victory.
Swan was in good touch with 28 runs off 42 balls but when Dwight was dismissed for 30 Western Bay were behind the eight ball.
The combined side were eventually dismissed for 161, to hand the defending champions a 43 run victory.
James Birchall backed up from a six-wicket haul on the previous day to take three wickets for Tauranga, with Scott Steward and Campbell Thomas also taking three wickets apiece.
Attrill Cup results on Sunday:
Tauranga 204/9 (Tom MacRury 68, Sean Davey 59; Jeremy Bates 3/32, Gavin Main 2/30) defeated Western Bay of Plenty 161 (Josh Dwight 30, Dale Swan 28, Ben Guild 21; Campbell Thomas 3/20, Scott Stewart 3/23, James Birchall 3/54)
Rotorua 135 (Bharat Popli 30, Rees Uerata 25; Matt Purdy 4/31, James Boyd 3/32) lost to Eastern Bay of Plenty 139/6 (Alex Yates 53no; Jason Trembath 2/24)
