Bishop Victoria Matthews, of the Anglican Diocese of Christchurch, yesterday said the cathedral would be taken down to a "safe" level of 2m to 3m so heritage items and taonga could be retrieved.
"The cathedral will be deconstructed with the utmost care and respect while at the same time protecting the treasure within its walls - there will be no bulldozers or wrecking balls on the job."
Rebuilding a replica cathedral had been ruled out because of an estimated $100 million shortfall. A new build incorporating some of the old would mean a shortfall of up to $50 million.
Critics attacked the church's decision to demolish as premature.
Christchurch city councillor Aaron Keown warned he would chain himself to the building to stop it being pulled down.
The Wizard of Christchurch, Ian Brackenbury Channell, described the decision as "an unforgivable act of barbarism".
Heritage campaigner Neil Roberts of the Christchurch Civic Trust argued more time should have been taken to consider options.
"Engineers from overseas who are experienced in earthquake strengthening have said 'Of course we can save parts of this building'. We don't have to deconstruct it or pull it right down. But they are not listening."
The Historic Places Trust said the building could have been restored or rebuilt in a recognisable form.
"The church's decision to deconstruct the cathedral down to sill level or a maximum of 2m to 3m around the full extent of the building will make it very difficult to retain any sense of this very important building as it once was," said chief executive Bruce Chapman.
But Earthquake Recovery Minister Gerry Brownlee praised the "courage" of those involved in the decision to bring the building down, while a sombre Christchurch mayor Bob Parker acknowledged it would not have been an easy decision for the church.
"It is not an easy decision for many of us to accept either."
Bishop Matthews said it would take the rest of the year to complete the deconstruction, and a temporary prayer garden might then be established at the site.
The Church was committed to establishing a "transitional cathedral" in central Christchurch after that.