The club had won its first premier banner for 56 years and only the second since it was formed in 1883.
Taieri won a hard-fought, and sometimes brutal, forward battle when it beat Harbour in a tryless final.
Craig Sneddon kicked two penalties from four attempts in the first spell to give Harbour a 6-0 lead at the break. But Ben Nowell won the game for Taieri by kicking four goals from five attempts in the second spell.
Harbour applied pressure from the start and led 6-0 after 22 minutes.
Pressure from the Harbour forwards caused Taieri to lose its structure in the first spell.
"We kept kicking the ball to them and they kicked it back to us and put pressure on our wings and fed off our mistakes," Taieri captain Charlie O'Connell said.
The tide started to turn in the last 10 minutes of the spell as Taieri started to get domination.
The onslaught was led by O'Connell at No8 - though he would blot his copybook by being sin-binned for 10 minutes for punching - and he was backed by hooker Will Hurst and prop Brett Anderson as Taieri camped inside the Harbour 22m and drove to the line.
It was only a determined Harbour defence that stemmed that tide.
Taieri played it a bit tighter in the forwards in the second spell and set up the attack two passes wider off the ruck.
The tactic worked because it stretched the Harbour defence and the players started to tire midway through the spell.
"I thought we were getting the ascendancy late in the first spell," Taieri co-coach Graeme Anderson said. "But they gave us a hell of a fight."
The game changed in the last 20 minutes when Taieri led 9-6 and Harbour had to play catch-up rugby.
Once the Eels gained the ascendancy, they were able to get front-foot ball, put pressure on Harbour and win penalties.
Harbour had played it tight in the forwards in the first spell but was forced to send the ball along the backline in the last 20 minutes in a desperate effort to get back into the game.
But the Harbour backs met a solid Taieri defensive wall spearheaded by Nowell, Shannon Young and Kieran Moffat and were not able to penetrate.
The tight forwards were the rock upon which Taieri built its championship win.
Lock Matt Larson dominated the lineout and made 10 clean takes. Fellow lock Kurt Webster was solid on defence and prop Brett Anderson was lively around the paddock.
But the two players who gave sting to the Taieri attack were O'Connell and Hurst with their fiery thrusts up the middle.
Fullback Kurt Schrader was safe under the high ball, made deep incursions into Harbour territory and was always dangerous on the counterattack.
Strong-tackling midfield back Lex Kaleca was the best of the Harbour backs, while No8 David Kahukiwa and Sam Ready won ball in the lineout.
Kahukiwa and Angus Duckett were lively in the loose and Pete Mirrielees and Ben Nolan were solid in the front row, and effective on the drive and with the pick and go.
The scores:
CLUB FINAL
• Taieri 12
Ben Nowell 4 pen
• Harbour 6
Craig Sneddon 2 pen
• Halftime: Harbour 6-0.