Sam Webb (three wickets for nine runs) and Sean West (3-33) did the damage with the ball for the northerners.
Stewart, who took 4-5 with the ball against Bay of Plenty Lakelands on Monday, topscored for Poverty Bay.
He faced 98 deliveries and hit five fours in his half-century. The Gisborne Boys’ High student showed a lot of patience but lacked support.
Northland reached their winning target for the loss of just one wicket.
Openers Luke Trigg and Lewis Miller put on 80 runs before Trigg was run out by Matt Hudson for 41 (off 46 balls), including five fours.
Miller carried his bat with an unbeaten 58 from 78 balls. He hit six fours.
Counties Manukau and Bay of Plenty Coastlands were the only two unbeaten teams going into today’s semifinals.