Christchurch's historic Catholic cathedral will be partially retained.

A $45 million cap has been set on the project to try and save the central space of the cathedral - the nave.

The building was set to be demolished, until renowned architect Sir Miles Warren was consulted.

Newstalk ZB's Tyler Adams was at the announcement and reports some areas won't be able to be salvaged.

"They will go into the Cathedral and assess the entire damage to that main area. What they so say is the areas outside the Cathedral, the sanctuary, that's all beyond repair so they've accepted that they won't be able to repair those aspect."

However, its future isn't guaranteed if the cost blows out.

Cathedral board chairman Lance Ryan says contractors will only get a real idea of the damage once they can get inside.

"it's got to be financially viable. When we get in it we could find it could be $100 million. We're budgeting on at this stage up to $30 million and our experts are confident that that may be achievable."

Lance Ryan says they will need to fundraise around $15 million.