The Volts posted a ground record 335 for seven with Michael Bracewell whacking 98 from 79 deliveries.
The visitors could only manage a paltry 113 in reply, gifting Otago its largest one-day victory.
All-rounder Sam Wells also played a key role with three for 15. Earlier he had combined in a 76-run opening stand with Aaron Redmond (63), getting through to 41 before he was given out caught behind.
Nathan McCullum helped push the score past 300 with 46 from 33 deliveries and took two for 16.
Perhaps the turning point came in the 26th over when Scott Kuggeleijn had both Redmond and Bracewell dropped. Bracewell was on 22 when Jono Hickey shelved a regulation catch on deep square leg boundary. It proved very costly.
Northern made an assured beginning but the departure of Mitchell Santner (15) with the team score on 47 triggered a rapid decline.
Daniel Flynn top-scored with 34 and Brad Wilson added 22 in an otherwise sorry innings.
