Suzie Bates and Katey Martin combined in an unbeaten stand of 122 for the second wicket to guide the Sparks to a comfortable win.
Bates, who was fresh from an unbeaten 150 in a one-day match on Tuesday, clipped 61 from 55 deliveries and Martin was even more assertive, smashing 65 from 47.
Earlier, Beth Mooney flayed 45 from 38 deliveries at the top of the order and Nicola Browne added 43 off 42 to help Northern post a competitive 133 for six.
Leigh Kasperek was perhaps the pick of the Otago bowlers with two for 23 from four overs. She picked up the key wicket of Browne. Morna Nielsen was almost as frugal, taking one for 25.
Victoria Abbott captured Mooney's wicket and her two for 28 was a very useful effort as well.
Otago's chase faltered early with Polly Inglis run out in the second over for one with the score on 13. But thereafter it was easy going for Otago's experienced pair on the Molyneux Park surface.
The winning runs were hit with 20 balls to spare and the victory has lifted Otago into second place with three wins from four games.
The Sparks last round-robin game is against the top-of-the-table Wellington in Dunedin on January 16 where a win will secure Otago a place in the final.
In the two other games yesterday, Canterbury had an eight-wicket win against Central Districts in Rangiora and Wellington beat Auckland by seven wickets.
