At least 75 people are known to have died, with the death toll expected to rise significantly, officials say.
Meanwhile, hundreds of foreign rescue workers are arriving to join the search effort, amid warnings that time is running out for survivors.
Authorities are working through the night to find survivors in an effort which has been hampered by aftershocks.Rescue work at one building, housing a TV network and a foreign language school, was called off as officials said there were no signs of life.
And the city's tallest hotel and surrounding area was cordoned off as rescue officials feared the hotel could fall on top of them.
The director of New Zealand's Ministry of Civil Defence and Emergency Management, John Hamilton, has said rescuers have only a window of two or three days to find people.