Councillors - with pleas from community members young and old ringing in their ears - voted 9-5 at yesterday's council meeting to declare the emergency.
They also voted to accelerate efforts to become a net zero carbon city, bringing forward the city's target for achieving the goal by 20 years, to 2030.
The vote came after people in a packed public gallery, many clutching signs and banners, cheered councillors who spoke in favour of action and laughed at those who resisted the move.
Cr Aaron Hawkins said the council had heard from ''countless'' people and organisations, over years, calling for action.
Progress had been too slow, forcing protesters on to the streets, and ''meanwhile, the clock is ticking'', he said.
''This needs to be at the front ... of all of our decision-making. A business-as-usual approach is not just inadequate; it's effectively inter-generational theft.
''This isn't about saving the planet. This is about saving ourselves.''
The vote was carried despite the opposition of Crs Lee Vandervis, Mike Lord, Andrew Whiley, Conrad Stedman
and Doug Hall.
Cr Jim O'Malley compared the initiative to the country's nuclear-free movement, which had also started small but grew to become a national movement with international clout.
Others agreed, including Cr Kate Wilson, who declared she would ''rather die trying than not try at all'', and Cr Christine Garey, who said she wanted to be able to look her own daughter in the eye after the vote.
Mayor Dave Cull also backed the move, saying the city needed to keep pace with the changing scientific consensus to avoid ''a point of no return''.
However, Cr Vandervis took a different view, saying that whether the scientific consensus on climate change was strong enough, and whether the council could do anything about it anyway, were fair questions.
''I don't believe we as a city council can do anything practical in that light,'' Cr Vandervis said.
Cr Lord also opposed the move, fearing labelling the situation an emergency was ''emotive'' and would only alarm people.
Cr Whiley accepted the climate was changing but questioned the process by which campaigners were shaping council policy.
Cr David Benson-Pope noted the debate had prompted the council's ''sceptics and deniers [to] out themselves'', but urged other councillors to act.
''I don't care if you call it a crisis or an emergency, because it's sure as hell both, actually.''
Earlier, members of the public took turns to urge the council to act.
Jennifer Shulzitski, of Extinction Rebellion Otepoti, said greenhouse gas emissions were still climbing, and, without action, the planet was on course to return to an ''unliveable'' climate, beginning with 2m of sea level rise by 2100.
''We have 10 years to make substantial changes and we are not doing anything near fast enough,'' she said.
''Like alcoholics, we need to admit that we have a problem.''
The loudest applause came when a group of four pupils from North East Valley Normal School delivered their own blunt assessment.
''The sea level is rising ... the ice is melting and we need to do something, fast,'' pupil Lochlie Jackson (8) said.