The visiting side slumped to 17 for three in the twenty/20 game but recovered to reach 108 for eight - a total which appeared to be at least 10 or 15 runs short.
Otago was seemingly cruising towards a comfortable win but was unable to lift the tempo in the dying stages and seal the win.
The captain, Katey Martin, was run out for 45 at the end of the penultimate over leaving Lisa Grant (27) and Natasha Miles (4) to score the remaining eight runs.
The pair scored seven runs to leave the game tied. Wellington's Sarah Taylor whacked the last delivery in the eliminator over for four to complete the win after Otago had managed seven for two from its extra six deliveries.
The loss was bitterly disappointing for Otago which had dominated for most of the match. It was also the province's third consecutive defeat to Wellington in three days with the visitors winning Friday's twenty/20 fixture by nine wickets and Saturday's one-day game by 43 runs.
Wellington scored an impressive 272 for eight in the one-dayer. Sparks star all-rounder Suzie Bates did her best with a superb innings of 92 from 76 deliveries. She found an ally in Martin, who scored 52 opening the batting, and Miles chipped in with 25.
But the Sparks batting line-up lacks depth and the team paid the price for some terrible bowling in which 27 wides and two no balls were conceded.
Sparks coach Nathan King said his team had shown signs of improvement over the last three days and should have been good enough to win yesterday's twenty/20 game.
"We just couldn't quite get the run rate down enough," he said.
"Between the 10th and the 20th there were just too many dot balls. I guess you can put that down to a mixture of execution and pressure. They [Wellington] had a couple of key bowlers who bowled through the middle stages and were able to put us under pressure."
The Sparks conceded 17 wides - that is almost three extra overs and it proved costly.
"In the end, little things like that do count against you. But we made a big improvement, both in the field and in terms of our discipline with the ball. But it is still an area we need to keep improving on."
Alana Kane was the worst offender, conceding four wides in her one over. She picked up the key wicket of Taylor, though. Taylor, of course, got to return for the eliminator over and had the last laugh.
Victoria Abbott was the pick of the Sparks bowlers with two for 12 from four overs.
