Waikato dominated much of a dour, forward-based encounter after Harbour suffered the setback of losing midfield back Shaun Treeby to a serious leg injury in the opening exchanges.
Up 6-3 at halftime, Waikato pushed eight points clear when Matthew Lansdowne towed ahead and scored in the tackle of Harbour fullback Matt Duffie.
However, Waikato winger Sevu Reece followed up and delivered a diving shoulder to the prone Duffie, earning himself a yellow card.
The try stood but Waikato's numbers disadvantage proved costly minutes later when Duffie scored a spectacular, diving try in the right corner.
Reserve back Ben Volavola converted from out wide but missed two penalties in the last 10 minutes which would have put his team ahead.
Fijian international Volavola put his hand up for another crack from a more difficult position after the final whistle sounded.
He made no mistake from 42m out on a wide angle to hand North Harbour a seventh win.
They are guaranteed to be ranked at least third for the semi-finals but a top-two spot is possible if they beat Taranaki in their last game next week.
- AAP