Tens of thousands of people were spending a freezing night without power after Christchurch was shaken by a series of violent earthquakes.
With temperatures plummeting under a clear winter sky, the Orion lines company warned more than 20,000 householders they would not be reconnected until at least Tuesday.
Residents were also being told to boil drinking water.
Christchurch Hospital said two of the people injured in the largest of the quakes, which measured magnitude 6.0 on the Richter scale, were still seriously ill.
Another 44 people were treated in the hospital’s emergency department, most of the injuries resulting from falling building materials.
Officials expressed astonishment that no one was killed in the tremor, the biggest aftershock so far of the 6.3 quake on February 22 in which 181 people died.
The Canterbury Earthquake Recovery Authority, set up by the government to manage rebuilding of the devastated city, said another 50 buildings had either fallen or would have to be demolished as a result of the latest shakes.
Christchurch’s landmark Anglican cathedral is also believed to have suffered further damage.
The tremors began at 7.37am on Monday and continued throughout the day, with the biggest striking at 2.20pm.
Panic-stricken residents, whose nerves have been stretched to breaking point by months of aftershocks, ran into the streets.
Some had difficulty standing as the ground rocked and swayed underfoot.
Ambulance and fire sirens screamed through the inner-city, with fire and shop alarms sounding, and clouds of dust billowing from falling buildings.
Rescuers pulled out two workers who were trapped in St John’s Church, in Latimer Square.
A shopping mall, supermarket, offices, a police station, and the University of Canterbury were among buildings evacuated.
The city’s international airport was closed for several hours while checks were made.
In low-lying eastern suburbs, vehicles sank into the road as liquefaction bubbled up, turning previously solid ground into sheets of muddy water before resolidifying.
Boulders dislodged by the shaking rolled down steep hills in some coastal settlements, narrowly missing houses and cars below.
Speaking just after the big quake hit, Bob Parker, the city’s mayor, said: “We are being enveloped with dust.
“It is very, very scary.”
He said the cordoned-off city centre “red zone”, where demolition workers were still removing rubble from February, was cleared after a magnitude 5.5 precursor quake struck at 1pm.
“Thank God we had evacuated the whole of red zone,” Mr Parker told Radio New Zealand.
“There’s been quite clearly a more significant building collapse in there.
“We are just now being enveloped by dust blowing out of the central city, we are just outside the cordon.”
John Key, the prime minister, promised citizens his government remained determined to support Christchurch, which already faces a NZ$15 billion (£7.5 billion) repair bill.
“We stand beside them, we are committed to rebuilding the city,” Mr Key said.
“Quite frankly, I think they’re all over this and they want the sense of normality to return.
“My heart really goes out to them.”
The opening day of an inquest in the city into the deaths of 106 people when the CTV building collapsed and caught fire in the February quake was abandoned for the day.