On Monday, the Supreme Court found the current voting age of 18 was unjustified age discrimination against younger Kiwis.
The bill of rights protects against discrimination on the basis of age for New Zealanders aged 16 and over unless there are reasonable justifications.
Over the past three years, activist group Make It 16 had sought a court ruling on that matter, and on Monday, the Supreme Court decided in their favour.
"This is history," Make It 16 co-director Caeden Tipler told journalists outside the court in Wellington.
"We are very optimistic that once parliament reviews the decision they will see that lowering the voting age is the right thing to do."
Owing to a recent law change, the ruling means both the government and parliament must now contend with the issue.
In August, Justice Minister Kiritapu Allan amended the Bill of Rights Act to require the attorney general to inform parliament of the inconsistency, triggering a debate.
The relevant minister, in this case Ms Allan, must also present a government response within six months.
It is unclear whether the court ruling will prompt the government to change the voting age, given the question is bound up in an electoral review reporting back next year.
Asked in a 2020 election debate whether she supported a shift to 16-year-olds voting, Prime Minister Jacinda Ardern said, "I wouldn't rule it out in the future but let's get civics right first, let's support our young people to learn about politics".
Youth turnout has grown markedly over the past decade.
Only 63 per cent of enrolled 18- to 24-year-olds cast a ballot at the 2014 election, which jumped to 69 per cent in 2017 and 78 per cent in 2020.
Those figures sit just below overall turnout, which was 82 per cent at the last election.
Ms Tipler said 16-year-olds deserved the right.
"At 16, you can work part-time ... in jobs that you're paying tax on. You can consent, you can learn to drive - all these things means at 16 it makes sense as the voting age rather than 18," she told Radio NZ.
The ruling marks the end of a multi-year fight by Make It 16 through New Zealand's court system.
In October 2020, a High Court judge declared the voting age of 18 was a "justified limit", and last year the Court of Appeal declined to rule, labelling it a quintessentially political issue".
Monday's Supreme Court finding overrules that judgment.
Opposition Leader Chris Luxon said his National party would not support changing the law.
"We've got to draw a line somewhere ... we're comfortable with the line being 18," he said on Monday.
The Greens, which already supported dropping the voting age to 16, labelled the decision a "massive win for democracy".
We are calling on the government to come to the table with a plan to change the law to extend the voting age," spokeswoman Golriz Ghahraman said.
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