A group behind an 11th-hour campaign to save an iconic Christchurch heritage building has admitted the decision to demolish the structure is "probably irreversible".
The 104-year-old Manchester Courts building was slated for demolition just days after it suffered extensive structural damage in last month's magnitude 7.1 earthquake.
Preparation work for the demolition began yesterday - the same day as a quarter-page advertisement in The Press newspaper called for work to be halted until an independent structural survey had been carried out.
The advertisement, backed by the Christchurch Civic Trust and the Icon residents group, disputed structural reports that found the building had only a "flimsy upper structure" above its reinforced concrete base.
Civic Trust chairman Tim Hogan said there was some conjecture the upper floors also had steel reinforcement, but more work needed to be done to establish it was there.
"The building has a structure that should be able to be strengthened and made safe. There doesn't seem to be any opportunity now to do that investigation," he told NZPA.
The group had received "quite a number of responses" to the campaign, but Mr Hogan admitted it was probably too late.
"At the end of the day it looks like it's probably irreversible at this stage."
The building's owner, Richard Peebles, said that while the building's upper floors had an internal steel frame, extensive testing and core drilling found no evidence that the external masonry had been reinforced with steel.
The building's columns had suffered severe bowing, which a University of Auckland expert told him was an "impending sign of failure".
Speaking from the demolition site, Mr Peebles told NZPA he would extend an invitation to Civic Trust to the inspect the evidence.
He said hundreds of thousands of dollars had been spent on restoring the building in recent years, including replacing lifts and polishing up wooden floors.
It was with great regret it had to be demolished, he said.
Southern Demolition director Alan Edge said preparation work for the demolition began yesterday, with the demolition proper to begin early next week.
Trucks were today delivering earth to construct a 6m to 8m bunding -- a structure designed to hold back breaches.
"That'll be formed up ready for the demolition which will start in earnest on about Tuesday," Mr Edge told NZPA.
The safety cordon around the building was yesterday extended by about 20m after strong aftershocks on Wednesday caused further damage to the building.
Those behind the campaign to save the building are hopeful other heritage structures can be saved.
"There are other buildings that are going to be assessed by the council in the coming weeks, so we're also intending to send a strong message to the council that we believe it's imperative that the heritage buildings are retained," Mr Hogan said.
"At the end of the day the heritage buildings in Christchurch are what defines us to a large degree within the central city, so somebody has to advocate for those buildings."
Council did not respond to calls for comment.