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The flag held a narrow lead in voting when preliminary results were revealed on Friday. Prime Minister John Key has said it's his preferred option.
In the official result announced this afternoon, which included all late and overseas votes, the Electoral Commission confirmed it as the winner.
The flag will now go head-to-head with the existing flag in a second referendum in March next year, which will decide whether New Zealand changes its flag for the first time in more than 100 years.
Both silver fern flags were designed by Kyle Lockwood, a New Zealander living in Melbourne.
The Red Peak flag placed third, the black and white silver fern placed fourth, and the koru flag fifth.
Under the preferential voting system, the blue and black flag won 50.58% of the vote in the fourth round of voting.
The second-placed flag won 49.42% of the vote.
The difference between first and second place was 15,000 votes.
The total number of voters in the postal ballot was 1,546,734, or 48.78% of the voting population.
The number of informal votes was relatively high at 149,747 - a sign that a large amount of people cast protest votes in opposition to a change of flag. This amounted to 9.68% of the total vote.
Deputy Prime Minister Bill English, who is in charge of the flag referendum process, said New Zealanders now had a clear choice whether to change the flag or keep the existing one.
"This is a historically significant choice we have in front of us. We now have time to consider the two flags side by side and have a good think about which best represents us as a nation now and into the future.
"I'd encourage everyone to have an input in this decision - even those who didn't vote in the first referendum, everybody eligible to vote can do so in the second, conclusive referendum."
The Government expected a stronger turnout at the second postal ballot in March, because New Zealanders would be voting on whether they wanted to retain the current flag.
The second referendum is from March 3-24. That referendum will bring to a close a process that officially began in October last year when Prime Minister John Key announced that a two-stage referendum would be held on the issue.
Close to 10,300 alternative designs were eventually submitted by the deadline in July.
In August, the long-list of 40 was released, and public interest and debate heightened when the final four were chosen by the Flag Consideration Panel in September.
A social media campaign for a discarded option, Red Peak by Aaron Dustin, gained traction, but calls for its inclusion as a fifth option were initially dismissed by Prime Minister John Key.
The flag was later added to the final list of flags after the Green Party brokered a deal with National and Labour to have it included.
- NZ Herald