Voters are being asked to choose between five flag options in a postal referendum that will continue until December 11.
The winning design will then go head-to-head with the existing flag in a second referendum to be held in March next year.
Prime Minister John Key has made the flag reform issue a pet project since his conservative government won a third term late last year.
He sees the current flag, with the Union Jack in the corner, as an anachronism, arguing the country needs a standard "that screams New Zealand".
Key has also expressed frustration the flag - which features four red stars representing the Southern Cross on a dark blue background - is frequently confused with Australia's.
Four of the five designs in the first referendum feature the fern, the informal national emblem.
The fifth, dubbed "Red Peak", consists of red, black and blue triangles with a white chevron. It was a late addition to the line up after a social media campaign for its inclusion.
An opinion poll last month predicted a design featuring a white fern on a red and blue background would win the first referendum.
But separate polling suggests the existing flag is likely to decisively win the second referendum in March with about 65 percent of the vote.
The present flag came into use in 1901, mainly because of New Zealand's patriotic fervour over sending soldiers to fight in the Anglo-Boer War in South Africa.
New Zealand was once part of the British Empire, and although it is now independent, Queen Elizabeth II remains head of state. However, her power is seen as largely symbolic, with many considering the monarchy itself as a colonial relic.
