Prime Minister John Key has been the main advocate for change, organising a referendum on the issue he describes as a once-in-a-generation chance to update the flag after more than a century.
"It's fundamentally about taking the Union Jack off and putting the silver fern on," Mr Key said this week.
He called the existing flag a relic of British colonial days, saying the silver fern used by the All Blacks "screams New Zealand" in the same way the maple leaf identifies Canadians.
But after an 18-month process costing NZ$26 million (AUD$23.18 million) it appears New Zealanders are overwhelmingly against change.
About 3 million ballot papers have been distributed in the South Pacific nation of 4.5 million people for the vote, conducted only by post and which closes at 7.00pm today (5:00pm AEDT).
Preliminary results will be released about 90 minutes later and polling has consistently indicated about two-thirds of the electorate support the existing flag.
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 stars.
Created by designer Kyle Lockwood, it beat four other proposed flags in a preliminary referendum last December.
Veterans' group the Returned and Services Association argues that to change the flag disrespects previous generations who fought and died under the banner.
Others criticised the design's aesthetics, with actor Sam Neill saying: "This ugly beach towel is no alternative. It's hideous."
But there are high-profile advocates for change, including ex-All Black skipper Richie McCaw, who said the existing flag was too similar to Australia's.
"The silver fern has always been the special symbol on the All Black jersey ... so the new flag with a silver fern as a part of it would be a great option," he posted on Facebook earlier this month.