About three million ballot papers are being distributed in the South Pacific nation of 4.5 million people for the vote, conducted only by post, which closes on March 24.
The result will be binding and John Burrows, the head of a panel overseeing the referendum, said New Zealanders would have to live with their choice far into the future.
"Whatever the decision, this flag will fly for generations to come and we hope all Kiwis exercise their right to vote in this historic decision," he said.
Key, a passionate advocate for a new flag, said the vote was a rare chance to update and modernise an important national symbol.
A supplied combination image of the final four alternative flag designs for a new New Zealand flag (AAP) Source: AAP
"If they don't vote for change now, they won't get another chance until we become a republic," he told Radio New Zealand this week.
"I don't think that's going to happen in my lifetime."
On one side of the ballot is the existing flag, a dark blue ensign with the Union Jack in the top left corner and four red stars representing the Southern Cross constellation.
On the other is the proposed alternative, a silver fern on a black-and-blue background, which retains the four Southern Cross stars.
Opinion polls point to a decisive win for the existing banner, with one survey this week finding 63 per cent wanted to keep it and only 26 per cent backed the new version.
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