The team behind Christ Church Cathedral Reinstatement (CCRL) says its costs have significantly increased, and the project may be mothballed if the funds cannot be found.
The landmark cathedral in Christchurch's city centre was badly damaged in the 2011 Canterbury earthquakes.
The cost of restoration was in 2017 estimated at $104 million. That rose to $154m in 2020, but a full project review has now priced it at $248m.
"This is an incredibly challenging position to be in, particularly when we are more than a third of the way through the entire reinstatement," reinstatement chair Mark Stewart said.
Gaining access to the inside of the cathedral for the first time last year gave a more accurate picture of what was needed, he said, and assumptions about the building's foundations were wrong.
Although the project's scope had been reduced, a significant cost increase remained.
"With the knowledge gained from the project review it became apparent that continuing the original project work plan would be too expensive and represent too much risk.
"On the recommendation from the project review team, the CCRL board has decided to reduce the scope, cost and risk of the project by removing the deep foundation for the tower and the lower courtyard, thus mitigating that risk."
Stewart said they were confident of fundraising another $26m, and the Anglican Church had offered $16m, "leaving a funding gap of $114 million".
"Philanthropy alone will not provide sufficient funds to complete the project."
The group had spoken to the council, mayor and Finance Minister Nicola Willis about the financial shortfall.
A report by the New Zealand Institute of Economic Research estimated the cathedral was worth about $20.8m a year in tourism spending.
"Mothballing the cathedral is something we hoped we would never have to contemplate - and we are optimistic of finding a solution to avoid this situation," Bishop Peter Carrell said.
"The cathedral would not have been built in 1881 without the full weight of the community behind it. It is now critical we demonstrate the same leadership as our 19th century Canterbury pioneers in returning the cathedral to life."
For the past decade, parishioners had been using the transitional Cardboard Cathedral. It was expected to be used until 2027, when worshippers would return to the traditional cathedral.
Mayor Phil Mauger said resolving the funding issue would take the combined efforts of the trust, the church, the city and central government.
"Many of our residents, businesses and visitors will see the Cathedral as the final piece of the rebuild," he said.
"But we must acknowledge that the financial pressure all of these groups are under at the moment will make this challenging."