The visitors led 17-10 at the break and appeared to be on track for a good win.
The front row of Rohan Wingham, Ricky Jackson and Saula Ma'u shunted the Harbour pack around.
And the lineout problems, which troubled Otago during the painful defeat to the Bay of Plenty, were long forgotten.
Harbour, though, brought some experience off the bench in the second half to help chip away at the lead.
And it was a brilliant play by dangerous fullback Shaun Stevenson which set up the winner. He ripped the ball free at the breakdown and got a nifty pass to Moses Leo, who crashed over.
Otago did their best to prevent him grounding the ball. But the slow motion replay revealed only horror.
He got it down.
That shapes as another critical match for the blue and gold.
The game started so well, too. They strung together phase after phase and lock Fabian Holland took advantage.
He swerved his big rig around a defender and crashed over.
But the home team responded brilliantly. Stevenson strolled through a tackle and tossed a well-directed pass to Sofai Maka out on the left to score.
Otago's set pieces were functioning beautifully and they snaffled a tighthead to set up their second try.
First-five Ajay Faleafaga made a snap decision to chip to the wing and Josh Whaanga collected the ball on the first bounce, stepped inside the defence and scored.
Harbour hooker Ray Niuia looked to have responded. He got across the chalk from a lineout move.
He would have netted five points but halfback Nathan Hastie got under the ball with a try-saving effort.
The backs grabbed the chocolates, though. Some slick passing saw the ball reach the hands of John Tapueluelu safely and he went over in the corner.
Harbour added a penalty just before the break and trailed by seven.
Otago kept their defensive intensity high and repelled several waves of attack.
Harbour got some reward through a second penalty to Oscar Koller.
But winger Jona Nareki scooped up a fumble from Harbour 40m out and took off towards the corner in his 50th game for the province. He got around the last defender and dived over.
The home side answered back with a second try to Maka. Leo had burst through a hole and got the ball to the speedy winger.
Then Stevenson snatched that turnover and swung the game.
Otago mounted one last effort. It amassed 19 phases before prop Jermaine Ainsley dropped it.
The scores
North Harbour 27
Sofai Maka 2, Moses Leo tries; Oscar Koller con, 2 pen, Bryn Gatland 2 con
Otago 24
Fabian Holland, Josh Whaanga, John Tapueluelu, Jona Nareki tries; Finn Hurley 2 con
Halftime 17-10 Otago
