The strong Kaikorai defence restricted Harbour to nine points in the first spell and then took control after the break in difficult conditions at Watson Park.
Harbour monopolised possession in the first spell but its only points came from three penalty goals by fullback Grant Sneddon.
Kaikorai trailed 9-0 at the break but quickly took charge in the second spell as its tightly knit pack took charge and made ground with the pick-and-go before sending the ball wide.
Lock Dave Simpson controlled the lineout, No 8 Adam Hill led the charge and hooker Rob Griffiths was skilled with the pick-and-go.
Two early tries by the fleet-footed backline took Kaikorai into the lead. Pacy winger Tony Ensor broke the defence for the first try by Jayden Spence, and first five-eighth Cam Rutherford beat four defenders for the second try by Ben Atiga.
Kaikorai played disciplined rugby and put the game out of Harbour's reach when flanker Lee Alan scored after a forward drive.
Harbour's frustrations came to a head when prop Pieter van der Merwe received a red card for punching.
- Kaikorai 24 (Ben Atiga, Jayden Spence, Lee Alan tries; Cam Rutherford 3 con, pen), Harbour 9 (Craig Sneddon 3 pen). Halftime: Harbour 9-0.