Australia has not done so, although the city councils of Sydney, Melbourne, Hobart and Adelaide, as well as the ACT government, have made a similar declaration.
"This declaration of the next generation ... it's about our global responsibilities. As a nation it's not in our nature to turn our backs on a problem," Ms Ardern said.
She said the new government pledge to be emissions neutral by 2025 was to "show leadership and demonstrate what is possible to other sectors of the New Zealand economy".
New Zealand's broader, country-wide pledge is to reach net zero emissions of all greenhouse gases other than methane by 2050.
That mid-century goal has been criticised by scientists and advocates as too slow to avoid irreversible and catastrophic changes to the climate.
New Zealand declares climate emergency
Oxfam NZ's Alex Johnston said "New Zealand has talked a big game about tackling climate change, but we still have a long way to go in achieving results".
"It's inexcusable that our country is one of the few industrialised nations in the world where net emissions have kept going up since 1990 - and that trajectory is not going to change much on current policy," he said.
Ms Ardern's landmark Zero Carbon Act, passed last year, created an independent Climate Change Commission which will set carbon budgets to guide NZ's emissions down.
Massey University's Robert McLachlan said it was sorely needed as NZ had "one of the worst climate records" among developed countries.
"New Zealand has put in place new institutions and mechanisms to cut emissions and to phase out fossil fuels. Now, we put them to work," he said.
The climate emergency motion was passed by NZ's parliament, with the National party in opposition declaring it virtue signalling and a "hollow symbolic gesture".