The final stage of voting in the New Zealand flag referendum has gotten underway.

Voters initially had until December to submit their postal ballots choosing between five new flag designs.

The winning design - Silver Fern (Black, White and Blue), designed by Kyle Lockwood - topped the poll with 50.58%, although the similar Silver Fern (Red, White and Blue) design won more first preference votes.

The successful Silver Fern is now going to head to head with the existing flag to see if the current flag will be retained or replaced.

The referendum is a postal vote, with voters able to submit their ballots between the March 3rd and 24th. An official result will be announced on March 30th, although preliminary results are expected shortly after polling closes.

While a new flag can be chosen by a parliamentary majority, authorities say "that a flag that unites New Zealanders should be selected by all New Zealanders".

New Zealand prime minister John Key, quoted by Stuff.co.nz, said, "I think these opportunities don't come along very often, they'll probably never happen again in the lifetime of most New Zealanders who vote in the referendum".