The Christchurch Earthquake Recovery demolition manager says a series of earthquakes on Monday have taken parts of Christchurch's inner city red zone back to what they were like in February.
Warwick Isaacs was with engineers checking indicator buildings in Lichfield Street when the quakes struck.
Mr Isaacs says up to 50 additional buildings within the zone have collapsed or partially collapsed.
The badly damaged 26-storey Hotel Grand Chancellor had moved further. A large area around the hotel has been cordoned off since the devastating 22 February quake.
Mr Isaacs says masonry and bricks fell all around him in Lichfield Street from buildings up to five storeys high.
Engineers will spend four or five days carrying out assessments and emergency demolitions may have to take place, he said.
He said there were up to 150 people in the red zone, most of whom were evacuated, other than Orion staff who were still working to get electricity on.
Superintendent Dave Cliff said at one stage there was a smell of gas in the red zone.
Buildings collapsed, weakened
Two people were rescued from a collapsed building in Hereford Street after the 5.5 quake and police said a building had collapsed on the corner of Stanmore Road and Worcester Street but was all clear.
The Dean of Christchurch Cathedral, Peter Beck, says the church has been further damaged.
The head of the Canterbury Earthquake Recovery Authority says says several buildings in the central city which were in need of demolition have collapsed on their own.
"There's buildings now that are seriously weakened that weren't actually damaged before this, said CERA chief executive Roger Sutton.
"We need to bring them down so we can safely access other areas. If anything, it may actually speed up some aspects of the recovery rather than slow them down."
Mr Sutton says the city is not "back to square one", as power should be back on within 24 hours and water supplies will be restored fairly quickly or water tankers brought in.