The All Whites were made to work hard in their Oceania playoff at Auckland's Eden Park on Monday, held goalless in the first half and losing captain Chris Wood to a hip injury early in the second.
Michale Boxall broke New Caledonia's resistance after an hour, the 36-year-old scoring his first international goal when New Zealand needed it most.
Kosta Barbarouses doubled New Zealand's lead soon after, allowing for a national exhale, before Eli Just wrapped up the result in the 80th minute.
"This is my fifth attempt to get to a World Cup," Barbarouses said.
"I'm really emotional, very happy, had flashbacks to the last 15 years.
"We're going to another World Cup, I'm esctatic."
The win means next year New Zealand will make their third appearance at the World Cup finals, benefiting from FIFA's decision to offer the tiny Oceania confederation a confirmed place at the 2026 tournament.
Led by Nottingham Forest striker Wood and with a generationally-talented squad, New Zealand thumped Fiji 7-0 in their semi-final on Friday and were expected to be far too strong for New Caledonia, the world No.152.
But for an hour at Auckland's Eden Park, that outcome wasn't clear.
The All Whites played without intensity or fluency, while New Caledonia goalkeeper Rocky Nyikeine - also in goal the last time Les Cagous beat New Zealand, in 2012 - was in fine form, denying the Kiwis at set pieces and in general play.
Defender Joseph Athale pitched in with a double clearance off the line in one frenzied backs-to-the-wall moment.
Despite his English Premier League pedigree, Wood never looked on song, even mis-volleying one effort backwards.
The All Whites looked flabbergasted at half-time as they walked off at 0-0, while New Caledonia's XI gathered in a circle - not quite celebrating, but their belief clearly building.
Johan Sidaner's band of amateurs and semi-professionals, almost entirely made up of either locally-based players or those from the lower tiers of French football, had never been one match from the World Cup before.
Sydney FC's Jaushua Sotirio, an unused substitute in both of New Caledonia's matches, was the sole pro, accepting a call-up to the nation of his parents' birth given the carrot of World Cup qualification.
Adding to their disadvantage, the New Caledonia Super Ligue has been paused since last year owing to major civil unrest which claimed more than a dozen lives and devastated the capital, Noumea.
Their stirring shift in Auckland came undone in the 61st minute, when substitute Frances de Vries sent a cross to the back post for Boxall to head home.
New Zealand's oldest squad member was joined on the scoresheet by the second-oldest, Barbarouses, five minutes later, effectively sealing the contest.
The 35-year-old raced onto a through-ball from Wellington Phoenix teammate Payne and chipped over Nyikeine, then turned provider for Just to thrash home a late third.
New Zealand ran out the match without their captain and vice-captain, with Wood off and Liberato Cacace needing a late substitution, though both were fit enough to join in celebrations.
Boxall said his emotion on scoring was "relief more than anything else".
"We made hard work of it," he said. "Fair play to New Caledonia."
Wood and Tommy Smith are the only current Kiwis to have played at a World Cup, the 2010 tournament in South Africa.
New Zealand lost playoffs to Costa Rica (2022), Peru (2018) and Mexico (2014) in their attempts to reach the past three World Cups.
While New Caledonia missed the direct qualification slot, they could still be the smallest and lowest-ranked nation to head to a FIFA World Cup.
They will be Oceania's entrant in an inter-confederation playoff next year, when they will reprise their underdog role as six nations compete for two places.
"Despite the loss, I love the way my team played, the attitude on the pitch. They gave everything," Sidaner said.
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