The Sparks captain struck seven fours on her way to 51 from 46 deliveries.
Martin teamed up with Leigh Kasperek (27) to put on 66 for the third wicket. Both players were dismissed with victory in sight.
Beth Langston chipped in with an undefeated run-a-ball innings of 11 to finish the job.
The England pace bowler had earlier been instrumental in restricting Northern to 126 for five. Langston took two for 28 and bowled the dangerous Bernadine Bezuidenhout.
Bezuidenhout had whacked 32 runs from 28 deliveries and was trying to swing away another boundary at the start of the final over when she perished.
Felicity Leydon-Davis top-scored for Northern with 35 and combined with Bezuidenhout in a partnership of 64 for the fifth wicket.
Kate Heffernan was the pick of the Otago bowlers. The left-armer took one for 17 from four overs, while Emma Black had a little luck in her haul of two for 23.
She captured Sam Curtis' wicket with a full toss which really should have been smashed over the ropes.
In the other matches, Wellington held on to beat Auckland by eight runs in a top-of-the-table game.
Wellington was dismissed for 103 at Lincoln Green. Auckland was in control at 70 for two but lost six wickets for as further 25 runs and was restricted to 95 for eight.
Off-spinner Eimear Richardson took two for 15 from four overs, and Lucy Doolan's three for 16 really put the brakes on Auckland's innings.
White Ferns leg-spinner Amelia Kerr took two wickets in the last over to seal the win for Wellington.
Canterbury beat Central Districts by seven wickets in a game also played at Lincoln.
Central posted 115 for eight but Amy Satherthwaite scored an undefeated 63 from 40 balls to clinch victory with more than four overs to spare.
