But former Blues pivot Michael Hobbs kicked a couple of penalties and when Blues flier Francis Saili scorched round his man to put winger Nafi Tuitavake away, Tasman looked unusually shaky.
A few minutes later, a break by centre Cory Aporo rattled them further, they snuffed out the raid near the posts but big Harbour lock Filo Paulo crashed over to build a 20-3 lead after 27 minutes. To say this was uncharted territory for Harbour this season is like saying Lance Armstrong once took aspirin.
Stung, Tasman stirred, led by strong-running centre Kieron Fonotia but Harbour's defence had magically sharpened itself up after the appearance of those 20 points.
Leggy replacement winger Peter Betham burst through to set up what seemed a certain try - but two things occurred. Tasman's final pass, delivered by fullback Tom Marshall, didn't match up to their earlier accuracy and a desperate defensive period by Harbour ended when they knocked the ball loose as Tasman lock Joe Wheeler tried to crunch over.
Betham got Tasman back into it - a quick lineout to himself and another leggy run worked space for James Lowe to go close and first five-eighths Hayden Cripps to finish.
But it was short-lived and you knew it was Harbour's night when a botched lineout saw the ball lob to new Blues halfback Bryn Hall. His speed over 45m took him away from the Tasman defence to seal the win, even with 20 minutes to go. Winger David Raikuna made certain with a try from a midfield play set up by Aporo.
Tasman will still get to the semifinals, albeit after travel. Harbour won't, but at least they finally put points on the board that didn't just come from being "nearly men''.
Tasman 10 (H. Cripps, try; T. Marshall con, pen), North Harbour 42 (N. Tuitavake, F. Paulo, B. Hall, D. Raikuna, V. Fihaki tries; M. Hobbs 2 con, 2 pen; C. Rei 2 con, pen). Halftime: 3-20.