Auckland Mayor Phil Goff says new COVID-19 cases in the community is something "none of us wanted to happen", but the lockdown is needed to stamp out community transmission.
Earlier on Sunday the Ministry of Health (MoH) revealed three new cases - a mother, father, and daughter from the same household in Papatoetoe.
Speaking at a press conference on Sunday evening with Director-General of Health Dr Ashley Bloomfield, Prime Minister Jacinda Ardern announced Auckland would go back into alert level 3 tonight, and the rest of New Zealand to level 2.
"We know from experience that quick action to impose restrictions is the best way to stamp out the virus," Goff says in a statement.
"Aucklanders have more experience than anyone else in the country in dealing with lockdowns. We know what to do to stop COVID-19 from spreading and we will do it again this time."
"As a city and as a country we must take a precautionary approach to maximize our opportunity to stop any spread of this virus, especially the new strains," adds Deputy Mayor Bill Cashmore.
"We have proven that it can be done, and that we can emerge safer and stronger."
Goff says he doesn't underestimate the impact another lockdown will have on people's lives and businesses around the city.
"I will continue to have discussions with government on the support our city needs during and after the lockdown to ensure we can continue our recovery," he says.
"I am urging everyone to follow Ministry of Health advice around good hygiene practices, social distancing and using the NZ COVID Tracer app.
If you have symptoms or you have been at locations at the time they were visited by the cases, make sure you get tested and avoid contact with others."
In line with government advice and when Auckland moves to alert level 3, Auckland Council's community facilities will be closed, including playgrounds and non-essential services such as libraries, pools and leisure centres.
Essential council services will continue, including rubbish and recycling and public transport services.