McCullum made 60 not out from 45 balls and Ronchi an unbeaten 48 from 25, boosting his credentials in the international game.
Ronchi has performed well as wicketkeeper but struggled with the bat in 50- and 20-over contests since declaring his allegiance to New Zealand last year. Loyalty from the selection panel paid dividends in a tricky situation. McCullum's presence to guide him appeared to help.
''I've felt good through the series, even without huge runs on the board,'' Ronchi said.
''Today it was nice to contribute and repay the faith the selectors have shown.''
The home side posted 189 for five, the third-highest total in 19 T20 innings at Eden Park. Wickets fell inconveniently after partnerships of 38 for the first wicket and 37 for the fourth. Jesse Ryder was mercurial, stroking the ball effortlessly, but that same instinctive play saw him caught at long-on.
The innings was in the balance at 104 for five in the 13th. McCullum could ill afford to lash out in his customary that's-the-way-I-play fashion for fear the New Zealanders would capitulate, as they did in the final one-dayer at Hamilton.
He chose the prudent option, enabling his team to build a defendable total on a wicket in its third use and appearing to slow as the match progressed.
McCullum was at his best playing late and working singles to good length deliveries from the spin-laden West Indies attack. That was not easy when he must have been tempted to force the pace.
His maturity was noted under the circumstances and made the difference.
''Every time I was just about to raise the momentum, we'd lose a wicket, so I'd try to get another partnership together,'' McCullum said.
''I wasn't as fluent as I'd like to be but there's so much power in our order that we can take another couple of balls.''
West Indian pace bowler Tino Best said it was still tough bowling to McCullum on a ground he found awkward.
''McCullum is difficult to bowl at. He's like Chris Gayle because he can demolish you. I think I bowled at him all right today but one of my slower balls was like 113kmh and he still whopped it high and 10 rows back.
''This is the funniest cricket ground I've played in. The straight boundaries are like club boundaries. Looking at this stadium, it's like they were more concentrated on the structure and they thought: 'Oh shoot, we'd better put in a cricket field'.''
The West Indies struggled to meet the required run rate from the 12th over, a task hindered by New Zealand's fielding. Martin Guptill's leap on the long-on boundary to catch Andre Russell will be another of his many catches to make the highlights reel.
Ryder's pace at long-off to pouch Dwayne Bravo was a credible rival. Nathan McCullum finished with four for 24 in his 50th match as a result. His lack of pace meant the visitors forced their shots.
This match was one of just two New Zealand plays in the T20 format before it heads to Bangladesh in March for the World T20. It provided an excellent snapshot of the team's potential.
New Zealand played almost its strongest line-up, the resting of Mitchell McClenaghan the only variation from what would have been a first XI. Adam Milne did the spot justice, getting a ball to angle in to catch the edge of Lendl Simmons' bat in his first over, closing in on 150kmh.
Milne went for one boundary in his allotment to finish with one for 15.
''It was nice to see 153 at one stage on the [speedball] radar,'' McCullum said.
''I'm pleased for Adam; he's bowled well without much luck for us in the past. Sometimes he is going to travel [to the boundary] but this was his best performance in a black shirt.''
The West Indies opted for a strong spin contingent of Samuel Badree, Sunil Narine and Nikita Miller on a pitch in its third use.
Useful cameos from Guptill (25 off 16), Ryder (22 off 14) and Colin Munro (22 off 12) were cemented by McCullum's early caution, enabling the freeing of the arms in the last six overs.
If there was an area for concern, it was the 18 dot balls conceded by New Zealand in the first 10 overs as it struggled to be decisive against the slower bowlers.
By Andrew Alderson, of the Herald on Sunday.
