The home side produced a bumbling effort in the field and leaked way too many runs.
Wellington posted 157 for seven. Melie Kerr made the most of an early life and pushed on to 56, combining with Leigh Kasperek (24) in a damaging 65-run partnership.
Otago’s chase unravelled rapidly after Katey Martin holed out for 40.
The Sparks lost their remaining six wickets for a further 15 runs and were bowled out for 91.
The side had strung together five consecutive wins before today’s 66-run loss.
They will not have much time to dwell on the poor performance, as they host Auckland at the John Davies Oval tomorrow.
Earlier, Otago’s Emma Black started poorly. She was getting prodigious swing but struggled to keep the ball from slipping down leg.
The wides were mounting quickly - way too quickly.
Wellington opener Georgia Plimmer took the attack to Molly Loe at the other end. She struck 14 runs off the second over but got greedy and hit a mis-timed lofted drive to Suzie Bates at mid-on.
Despite losing a couple of wickets, Wellington had rattled on 57 runs in the powerplay.
The Sparks' fielding was really sloppy, but veteran keeper Martin made a sharp catch at the stumps to remove Maddy Green for seven.
But the dropsies resumed. The in-form Kerr hit an easy caught and bowled chance to Black.
Kerr was on 15 and her departure would have been a seismic shift. But she survived and went on to forge the partnership with Kasperek.
The Sparks had opportunities to secure a breakthrough. A couple of run-outs went begging and Martin missed a stumping.
Kerr brought up her 50 off 43 balls, but eventually ran out of luck. She shuffled across the stumps but was bowled by Sophie Oldershaw.
The wrist spinner finished with one for 19 from four overs, which was a tremendous contribution in the context of the innings.
The last five overs netted just 36 runs at a cost of three wickets.
Martin banked consecutive stumpings and Otago did well to restrict Wellington to under 160 in the end. But a bungled run out chance and a dropped catch in the last over summed up a poor fielding display by the home team.
The Sparks also conceded 14 wides which slots in comfortably at the unimpressive end of the spectrum.
Otago’s chase faltered early. Polly Inglis got herself bowled for one.
But that brought the old firm of Bates and Martin together and the partnership held the key for the Sparks.
Otago was well behind at the first split, though. The powerplay netted just 32 for one.
Martin had a life on 19 and that was a big missed opportunity. But the visitors got gifted the wicket of Bates for 24 after she got too deep in her crease and stood on her stumps.
And when Martin holed out for 40, the celebrations could begin in earnest.
The Sparks had fallen too far behind and did not have the hitting power left to threaten the tally.
