Harbour led from almost the opening whistle after Sione Mafileo crashed over from close range to capitalise on an early error from the opposition, the first turnover of many for a sloppy Counties attack.
The home side responded well and began to enjoy prime field position but they coughed up possession far too often to mount any serious pressure on the Harbour defence.
They would have headed to halftime in a serious hole but for an extremely late break from Tim Nanai-Williams, setting up Luteru Laulala's try almost three minutes after the hooter had sounded. And some more Nanai-Williams magic put his side in front shortly after the break, crossing for his own try to build an unlikely lead.
But Volavola soon put Harbour back ahead and, after spending much of the final quarter deep in Counties territory, Li finally finished it off.
Counties-Manukau 18 (L. Laulala, T. Nanai-Williams tries; L. Laulala 2 pens, con)
North Harbour 27 (S. Mafileo, T. Li; B. Gatland con, pen, B. Volavola 4 pens)
Halftime: 11-16