The Anglican Cathedral sustained further damage in the magnitude 5.2 aftershock which hit Christchurch on Friday afternoon.
The church says the building is beyond repair and work has begun to reduce the structure to just two or three metres in height.
The decision has been controversial, with a group fighting for the building to be rebuilt.
A church spokesperson says that since Friday there has been more noticeable bowing in the walls, especially towards the southern end, and more stone has fallen.
The church says the plans for taking down the walls have to take the ongoing seismic activity into account.
It will issue the methodology for the deconstruction as soon as it is approved.
The earthquake occurred at 2.44pm on Friday. It was centred 10km east of the city at a depth of 11km.