Harbour adapted better to the requirements of wet-weather rugby when Dunedin was beaten 19-10 at Watson Park.
Harbour played it tight and kept the ball in front of the forwards who drove into the breakdown as a cohesive unit.
It was smart tactics in the conditions and its grafting approach led to three tries to give Harbour a 19-3 lead at the break.
Harbour dominated the scrums with powerful play by front-rowers Pete Mirrielees, who was aggressive and mean, and Pola Finau.
It also had the better of the lineouts with lock Brad Griffiths getting his share of ball.
Dunedin was on top for the first 10 minutes of the game but surrendered the initiative to Harbour when loose passing led to an intercept by midfield back Lex Kaleca who raced 50m to score the first try.
Weak tackling allowed wing Rema Smith to score Harbour's second try midway through the spell by running down the blindside.
Dunedin played better in the second spell, had time on attack and its defence shut Harbour down.
But the brutal Harbour defence kept Dunedin out until loose forward Gareth Evans scored late in the game.
Harbour 19 (Lex Kaleca, Rema Smith, Pete Mirrielees tries; Craig Sneddon, Ben Main con), Dunedin 10 (Gareth Evans try; Liam Edwards con, pen). Halftime: Harbour 19-3. Referee: Jeff Grubb.