A St John spokesman said an ambulance was sent to the Hotel Grand Chancellor site in Cashel Street, within the red zone, at 8.10am.
"We took one patient, a male aged approximately in his 40s, with very serious injuries to Christchurch Hospital Emergency Department."
The spokesman was unable to comment on how the injuries were sustained.
Fletcher's chief executive of construction, Graham Darlow, said: "Our thoughts are with the worker and his family.
"Right now, our focus is on ensuring that they have all the support they need. We are really distressed that one of our people has been harmed, and feel deeply for the family."
Mr Darlow said Fletcher Construction had notified the Department of Labour of the accident.
The worker's family was now with him, Cera said.
Cera general manager, operations, Warwick Isaacs, assured the public that Cera and Fletcher Construction took workplace safety very seriously.
"Together, CERA and Fletcher Construction developed a sound and robust safety plan for the demolition of the Hotel Grand Chancellor.
"It is distressing and concerning that an incident resulting in serious harm has occurred.
"Fletcher Construction acted immediately to get help for the injured worker, ensure no further people were at risk and to check on the wellbeing of those who witnessed the accident."
Mr Isaacs added: "Out of respect for the worker and his family, and because we all want this fully investigated, I am unable to provide any further details at this time."
