New Zealand had looked to be wobbling when they were reduced to 79 for five in the 11th over chasing 160 for victory, but wicketkeeper Ronchi powered them over the finishing line with an unbeaten 51 off 28 balls that included four fours and two sixes.
Ronchi also scored a 25-ball 48 as New Zealand won the first match by 81 runs in Auckland.
On this occasion he was ably supported first by Ross Taylor (39) and then by James Neesham, who crashed all 14 of his runs in the space of four deliveries in the penultimate over from Tino Best as New Zealand won with six balls to spare.
Earlier, the West Indies made 159 for five after winning the toss and opting to bat first at Westpac Stadium in Wellington.
On a good day for the wicketkeepers, Windies gloveman Denesh Ramdin top-scored with 55 not out off 31 balls from number six, striking three sixes and as many fours, while Andre Fletcher (40) and Lendl Simmons (29) both made useful contributions.
And Ramdin's late blitz - he smashed 16 off four balls in the 16th over from Corey Anderson - looked set to be decisive as New Zealand lost several of their big-hitters inside 10.3 overs with Jesse Ryder (23), Brendon McCullum (17) and Anderson (six) among those departing.
However, Taylor remained and he shared in a 68-run stand with man of the match Ronchi that swung the contest back in New Zealand's direction.
Taylor was unable to see it through to the end as he was caught at deep midwicket off Sunil Narine (two for 18) having scored 39 at around a run a ball, but that breakthrough only helped speed the West Indies to defeat as Neesham went on the offensive against Best.
- Sports & Recreation
- New Zealand
- Ross Taylor
- James Neesham