"The situation meant we needed to bat for as long as we could," Ronchi said. "To get a hundred and push to a score that was defendable provided a lot of fun and joy."
"We were pretty lucky to evade the first 12 overs which looked quite spicy," added Elliott, who didn't know which world record they'd broken.
"It was fun to be out there with Ronchs. I was just sitting at the non-striker's end trying to get the strike off him but I couldn't. He kept hitting fours and sixes.
"Fortunately the way he was playing I could push ones and get us into position so it was building a foundation - and that's my role so we can strike later."
The stand overtook the 218 world record mark set by Mahendra Singh Dhoni and Mahela Jayawardene playing for the Asian XI against the African XI in 2007 at Chennai.
It was the second best partnership by New Zealanders against any country behind the opening effort of 274 by Brendon McCullum and James Marshall against Ireland at Aberdeen in 2008.
The 360 for five total is the sixth highest score by New Zealand in ODIs.
Ronchi savaged 170 off 99 balls, continuing the form he found to deliver 24 off the 48th over to help win the game in Nelson. It's the highest score by a No 7 batsman in ODIs and fourth highest by a New Zealander. His bat speed dominated the wayward bowling.
Elliott played the anchor before tapping into a seam of aggression which took him to his second ODI century of 104 off 96 balls.
He rotated the strike effortlessly, further justifying his World Cup selection as a No 5 option.
A rare escape came at the start of the 35th over when Elliott, on 47, hit a shot to cover and an outstretched Lahiru Thirimanne bobbled it on the ground. Ronchi had two lives with dropped catches on 95 and 167.
Elliott also bowled steadily, taking two for 42 in nine overs as captain McCullum asked him to take more all-rounder responsibility across the game. Trent Boult justified his recall, taking a career-best four for 44 from his allotment, while McClenaghan took two for 56 from eight.
Sri Lankan opener Tillakaratne Dilshan led a spirited chase as he eased to his 20th ODI century, 116 from 106 balls, but the pressure of chasing more than 10 runs an over was overwhelming. He exited with the score at 215 in the 37th over trying to loft a short ball from Mitchell McClenaghan over the offside. He spooned to Nathan McCullum at extra cover, precipitating a collapse in which they were all out for 252.
New Zealand were sent in by captain Lahiru Thirimanne, who was standing in for Angelo Mathews as he nursed a calf muscle injury. It's uncertain whether Mathews will return tomorrow for the sixth match of the series, also in Dunedin.
