An earthquake measuring 7.4 on the Richter scale hit Christchurch and other parts of the South Island.
It was felt as far North as Taranaki and has caused widespread damage to buildings, roads, and power has been cut to most of the city.
As yet, there are no reports of serious injury or death.
Striking at 4.35am this morning, the epicentre of the quake was near Darfield, 30km west of Christchurch and it struck of a depth of 33km underground.
Civil Defence is asking residents to not use their cell phones, except in an emergency, and to refrain from driving about the city. Because of the cut to power, many cellphone towers have switched to a battery powered backup. Many roads have been closed.
Minister for Civil Defence John Carter has asked people to remain calm.
There has been looting in the city, including the smashing of windows, and a Police spokesman has stated that some arrests have been made.
Christchurch Airport remains closed until further notice at 12pm.
Update: The earthquake has been downgraded to a magnitude of 7.1.
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ENDS