McKenzie nailed the handy conversion before his calm late heroics.
Again it was Southland who kept their composure and when they got in the scoring zone, they did not hesitate. The ball went back to McKenzie, who steered the ball between his target. A rare away win in the deep south and then a probable draw slipped away for Harbour in a numbing late episode.
"It hurts, it really does hurts," captain James Parsons said. "We started well and then slipped off our work."
Five-eighths Jon Elrick kept Harbour's score ticking with his brisk goal-kicking routine. He missed just twice in his seven attempts and, with his pack working strongly, was able to keep the tempo up with his boot and distribution to the dangerous Ahki, Francis Saili and David Raikuna.
Referee Mike Fraser penalised Southland heavily for their lack of discipline and adherence to the revamped scrum laws in the first half.
When they returned with their coaches' words burning their ears, Mckenzie scored inside a minute to start a four-try comeback and eventual triumph.
Southland (Marty McKenzie 2, Cardiff Vaega, Hale Te Pole, Tim Boyes tries; Scott Eade con, Marty McKenzie 2 con, dropped goal )
North Harbour (Jordan Manihera, Bryn Hall, Pita Ahki, tries; Jon Elrick 2 con, 4 pen) Halftime: 16-5