It was Tasman who ruled the defensive roost by stepping up their own intensity to restrict Auckland's chances to play the game they wanted, especially denying them the ability to get into positions to maximise their try-scoring potential.
There was no way through for Auckland, a fact demonstrated during a crucial period of 17 phases, hot on defence, proving a blunting force that marked the difference on the night.
It was a classic, tight, final, compounded for Auckland by the loss of skipper and prop Angus Ta'avao to the sin-bin moments before halftime and hard on defence.
That was enough for Tasman to pounce, unleash their lineout maul with Auckland one man short, and in the wash-up it proved the key moment of the game as hooker Quentin MacDonald claimed the try.
Into the second half, Auckland attempted to prise open the Tasman defence but there was no way through and the difficulty of their cause was hit home in the 77th minute when substitute Simon Hickey was unable to find the posts with a dropped goal attempt.
And from the 22m drop out as Auckland attempted to run the ball back, it was fullback and skipper David Havili who produced a vital turnover when centre Tumua Manu was ruled to have held on too long in a ruck.
Tasman won the penalty, won the resulting lineout and halfback Finlay Christie had a look at the clock, heard the fulltime hooter while running back several metres before kicking the ball out to seal the win.
Auckland also had another period without their full complement when fullback Zarn Sullivan, who late in the first half had landed a superb penalty goal from 52m out, was sin-binned for a deliberate knock down as Tasman replacement back Keiron Fonotia attempted to slip a pass to block-busting wing Leicester Fainga'anuku.
Fonotia had been on the field since the 14th minute when first five-eighths Mitchell Hunt had to leave the field after being hit in a solid tackle by flanker Niko Jones.
While that might have been a problem for most sides, especially in losing their goal-kicker, David Havili stepped up and landed two penalty goals and the conversion of MacDonald's try. It was a superb demonstration of leadership under pressure.
Equally, Auckland were unable to get the ball to their key assets on the wings Salesi Rayasi and AJ Lam.
Tasman's pack was outstanding. Blindside flanker Hugh Renton contested everything at the breakdown pulling off some key plays while also a nuisance in the lineout. Lock Quinten Strange also pulled in some key ball at lineout time.
David Havili's contribution was significant with, and without, the ball while halfback Christie provided a savvy display in such a tight contest.
Tasman were also untroubled, in spite of having a try to centre Fetuli Paea ruled out. He went in at the corner off a fine pass as No8 Taina Fox-Matamua capitalised on another attack started with Havili fielding an Auckland kick and charging through the Auckland defence to open space.
However, in the grounding of the ball, it was a hairline decision for the TMO to find that Paea's back had touched the sideline a fraction before he put the ball over the line.
Auckland managed to clear the line after Tasman won the lineout, but they were unable to score the points that would have made the game safe.
Scorers: Auckland 12 (Harry Plummer 4 pen) Tasman 13 (Quentin MacDonald try; David Havili con, 2 pen). HT: 9-10