Wellington were all out for 115 in 41.1 overs. Their top scorer was No 8 batter Isla McKenzie, with 32 runs off 70 balls.
Northern Districts have competed in women’s domestic cricket since November 1999 and had appeared in the Hallyburton Johnstone Shield final just once before, in March 2020, when they were beaten by the Auckland Hearts.
This summer, Northern Districts lost only one match in the regular season and finished 10 points clear of the field.
After Northern Districts skipper Jess Watkin won the toss and elected to bat, Wellington looked to have gained the upper hand when they took key early wickets, including that of Watkin, given out lbw in the third over.
But the lower order gave Northern Districts a decent total to defend and they then set about dismantling the Wellington batting line-up.
Downes took 3-26 off 10 overs, two of them maidens, and Watkin took 2-25 off 10, one of them a maiden.
The White Ferns face Zimbabwe in the first of three T20s in Hamilton on Wednesday.