South Library is the last of the council's libraries to be rebuilt or repaired following the Canterbury earthquakes.
The library's current seismic rating is 34 percent - any lower and it would be classified as an earthquake-prone building.
The projected cost to repair the library was $26.6m, while the rebuild option was $24.9m.
Spreydon-Cashmere Community Board chair Karolin Potter was among residents who fought for the library to be built on its riverside site over 20 years ago, when the council was considering selling off the land.
The library had almost become an integral member of the community, she said.
"I couldn't emphasise enough how much we feel about that library and how much like a person it is. When you go in there, you - honestly, it sounds silly - but you go in there only thinking of the positive things that have happened in that building," Potter said.
It was also the first in the city to include a cafe, computer room and service centre under one roof.
The library had temporary strengthening carried out a year after the earthquakes in 2012, but Karolin Potter said it's no longer enough.
"I know the community feels a lot about this library ... but [the rebuild] has to happen because if the library's percentage goes any lower, it's uninhabitable. So you can't just stand around and say 'we'll keep the props going for ever and a day'."
She was pleased it would be built back on the same site, but warned the council would need to consult extensively with the local community on the new building.
South Library will stay in operation until work begins, which is expected to be late next year.
The rebuild's funding is yet to be addressed by the council but so far only $13.6m has been set aside for the project.
RNZ
