As many as 22 tourists are thought to be buried in the rubble of Christ Church Cathedral and its spire.
The Bishop of Christchurch, Victoria Matthews, said staff, including Dean Peter Beck, escaped the building but authorities believed up to 22 people could be still under the wreckage.
Hundreds of people visit the 130-year-old cathedral every day - most of them tourists - and it is feared many were inside when the 63m spire collapsed in the quake.
"The staff are okay, the volunteers are okay, the ones we're most concerned about are the people who had gone up into the tower - this happens with tourists every single day," said Bishop Matthews.
"They go up because there is - or there was - such a fabulous view from the tower."
"The woman who got out about one minute before the tower came down said there were people up there behind her when the tower came down.
"We don't know about bodies under the rubble and we're very, very concerned, very much considering that there will be fatalities."
Jamie Canard was eating lunch and watching people play a game on the giant chess set in the city square when the quake hit.
He watched the spire crumble.
"As soon as it fell, I knew people would be dead."
The 38-year-old programmer ran into the cloud of dust created by the collapsed spire to try to help those who were trapped or injured.
But someone yelled: "Leave! Get out! It's not safe."
He said he knew people would be buried under the rubble, but couldn't see anyone.
"I now feel like a fool leaving, now that I know people died trapped in there."
Mr Canard described the spire's collapse as a steadily increasing crescendo.
"It wasn't deafening, which surprised me, but it just slowly got louder."
He said the tower "just toppled over".
Another area of concern was the south porch, which fell into the building.
Although Bishop Matthews said those inside were likely to be tourists, anyone could have been in there.
"Because it was iconic and they would have been in there for a number of reasons, just to go and see it or to come and pray, or perhaps for the noon Eucharist."
She said church staff were compiling a list of Christchurch churches that were safe enough for people to come to pray, or to be used for funerals.
The church was focused on offering its safe churches for funeral use, as well as welfare centres where people could come for coffee and counselling.
The number of damaged churches was not known, but was significantly more than the 12 damaged by the September 4 quake, she said.
She said the cathedral would be rebuilt.
"It's very, very serious damage but it will be rebuilt. We don't see any reason why that will not happen.
"We have every reason to think it will be fine."
Dean Beck said on Wednesday he still did not know how many people were killed "but many will be tourists because the cathedral is such an important visitor attraction to the city".
Police sniffer dogs had found no sign of life in the rubble, and he accepted that the priority for emergency services was to rescue those who may yet have a chance of survival amid the collapse of other buildings.
"The focus of the police is to get the living out - so the main priority at the moment is not to get the bodies out."
The cathedral has until now attracted about 700,000 visitors a year and climbing the 134 steps of its bell-tower was one of the most popular of the many activities it had to offer.
The tower was 36m high and the spire rose 27m above that.
Tower climb certificates were available for those who made the ascent.
From the top of the spire, visitors gained spectacular views of Christchurch and the Port Hills.
Much of Christchurch was built around the cathedral, which was conceived of soon after the first four ships of European settlers arrived in 1850.
Its foundation stone was laid in 1864, and it was eventually completed in 1904 at a cost of £64,000.
Although it was designed according to the tradition of European cathedrals, ceiling timbers of matai and totara from Banks Peninsula, stone from local quarries and Polynesian art works gave it a strong cross-cultural identity.
* A service will be held at midday today for anyone wanting to attend, at St Peters Anglican Church in Takapuna.