Harbour had sound lineout options in Chris Vui and Gerard Cowley-Tuioti, but hooker James Parsons could not always find them.
Matt Vaega, who kicked two goals, and Michael Little, both punched holes, but the Steelers will not crumble defensively, especially not with Stuart Lancaster pulling the strings in that facet. Instead, Little pounced on a Vaka error in his in-goal for the second Harbour try from a grubber kick.
Replacement halfback Chris Smylie racked up his 50th game for North Harbour, 14 years after his debut.
The Steelers will look forward to the offensive spark of their returning sevens pair - August Pulu and Sione Molia - next week.
Both sides have plenty of time for work-ons before their next, crossover, matches. North Harbour have nine days before they face Waikato in a Ranfurly Shield challenge on August 27, while Counties Manukau host Bay of Plenty the following day.
The curtainraiser saw Westlake BHS triumph 34-3 over Massey High to win the union's First XV final for the 15th time.
North Harbour 20 (Afa Fa'atau, Michael Little tries; Matt Vaega con, pen, Bryn Gatland con, pen) Counties Manukau 17 (Jordan Taufua, Suliasi Taufalele tries; Piers Francis 2 con, Joe Reynolds pen) HT: 10-10