Left wing Tevita Li looked to have fluffed it on his edge, but a cool draw and pass from fullback Matt McGahan put Matt Duffie over for their first.
A massive Uluiviti fend on Duffie looked to have put wing Monty Lama away but some quality cover defence from Harbour saw off the threat.
Second five Dan Hawkins became the victim of too many penalties as a coathanger tackle resulted in a yellow card.
Harbour finally broke the Taniwha defence as they spread the ball wide. McGahan nearly crashed over but got the ball away to Li for the converted try.
Li could have been in five minutes later if not for a try saver from Alaimalo. But it didn't stop them for long as quick hands saw Duffie in by the corner post for his second.
A knock on from Hawkins a minute later led to a 60 metre runaway try for second five Michael Little, who had too much pace for the covering defence.
Gatland added his second successive sideline conversion to give Harbour an imposing 32-7 lead at the break.
The back three of McGahan, Duffie and Li were showing the threat they pose with consumate ease.
After a fairly quiet start to the second spell, Harbour scored after a possession saw the ball go the length of the field three times.
After turning the ball over after a Harbour siege, Shane Neville put through an ill-timed kick to McGahan. The fullback linked up with Li who found acres of space, who in turn found Connor Collett for the five-pointer.
Poor execution was plaguing Northland, who couldn't control the ball in the Harbour red zone.
But they finally broke through. Multiple pick and go's ended with replacement hooker Ross Wright burrowing his way over to give the Taniwha faithful something to cheer about.
A yellow card to Afa Fa'atau gave them another opening, which Alaimalo took full advantage off by cruising over the line for a well deserved try.
Despite the usual strong performances from Matich, Breen, Alaimalo and Kara Pryor, it wasn't enough for the home side.
Replacement prop Karl Tu'inukuafe then bowled over to give Harbour a 23-point buffer.
Wisea Lawebuka gave Northland their bonus point try but the whole they dug themselves into proved too deep to climb out of.
Captain Matt Moulds was proud of the fight from Northland, but lamented their first half effort.
"Awesome second half, it was a real game of two halves. Their backs were too slick in the first half but a good fight back from the boys," he said.
"We started well but the little things cost us, much like the rest of the season."
North Harbour move up to third in the Championship, setting up a semi-final date with Wellington.
North Harbour 44 (Matt Duffie 2, Tevita Li, Michael Little, Connor Collett, Karl Tu'inukuafe tries, Bryn Gatland 3 con, 2 pen, McGahan con)
Northland 28 (Epineri Uluiviti, Ross Wright, Solomon Alaimalo, Wisea Lawebuka tries, Peter Breen 4 con)
HT: 32-7
