New Zealanders have begun voting on whether to change their flag.

The postal ballot runs from Thursday to March 24, and offers voters a choice between the existing flag featuring the Union Jack and the alternative silver fern design.

A previous referendum selected the Kyle Lockwood-designed fern flag as the most popular among five alternatives.

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Opinion polls indicate the nation of 4.7 million people will opt to stick with its current flag, although proponents of the new design say they have momentum on their side and that more and more people are embracing a change.

Prime Minister John Key says the vote is a once-in-a-lifetime opportunity.

“It will probably never happen again in the lifetime of most New Zealanders who will vote in this referendum,” he said on the eve of the vote.

“Even though it makes the process much harder by asking the public what they want, personally I think that’s the right thing to do.”

Mr Key wants change and wears the silver fern flag on his lapel.

He says if the referendum goes against him, he will not take it personally.

“Not in the slightest … I think it’s been a pretty healthy debate about New Zealand and our nationhood,” he said.

“And I hope people will vote for the right reasons – if they vote because they don’t like me, they will be wasting their vote.”

Labour, although it proposed changing the flag in its 2014 election manifesto, opposed the referendum.

So does NZ First, and leader Winston Peters has urged people to spoil their ballot papers by scrawling KOF (keep our flag) across them.

The Electoral Commission will start mailing out the ballot papers on Thursday, and had urged people to enrol before the Wednesday deadline.