Captain Michael Bates was out off the penultimate delivery from Anurag Verma, with two needed.
In Dunedin, leaders Central Districts got up in a tense finale over Otago, courtesy of an unbeaten 114 off 115 balls by Kruger van Wyk.
Chasing Otago's 255, CD were in difficulties at 60 for four.
However, van Wyk and Dane Cleaver put on 112 for the fifth wicket and CD won with five balls to spare.
A big hundred from Henry Nicholls, with a Canterbury List A record 178 off 161 balls, helped lay waste to bottom side Wellington.
Nicholls shared a 291-run second wicket stand with Neil Broom, who hit 117 at better than run-a-ball rate and pushed Canterbury to their third highest 50-over total, 356 for three.
Veteran James Franklin collected 102 in 87 balls, but Wellington were no chance, being dismissed for 271, losing by 85 runs. Stephen Murdoch had been Wellington's other hope, but he hit a long-hop straight to the fielder at square leg on 55.
Points going into the final round: Central Districts 26, Auckland 24, Otago 15, Northern Districts 12, Canterbury 10, Wellington 5.