A magnitude 7.1 earthquake struck about 30km west of Christchurch at 4.35am today, leaving extensive damage in the city.
The quake was origninally reported as 7.4, but was later downgraded.
Emergency services have been inundated with calls for help following the quake which had a depth of 10km.
Police said power to Christchurch was out and sewer lines had been damaged, and that part of Avonside Dr had been damaged and could not be driven on.
Structural damage to houses and broken glass was being reported from throughout Christchurch, while damage and power outages had been reported from as far away as Dunedin.
A Timaru resident told NZPA he was awakened by the shaking. His house was groaning and a bedside lamp crashed off its table.
A man who lives in the southern Christchurch coastal suburb of Southshore described the quake as "incredibly long and tortuous".
He was on the second floor of his house and could feel the house "twisting and fracturing around our ears", and it had been left "on a bit of an angle".
Photographer David Alexander, of suburban Opawa, told NZPA the aftershocks "just keep coming". His house still had power but street lights were out.
Alexander said his wife was from Japan and had experienced worse shocks but she said this was a pretty good one.
Their 103-year-old villa had withstood the onslaught well but there was broken glass and china everywhere, and "a very frightened cat under the bed".
Christchurch man Chris Hutching said the water mains had burst
in his St Albans neighbourhood and that "chimneys are down all
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Streets and Airport closed
Radio NZ said police had shut down the central city for safety.
Christchurch Airport was closed and officials were checking the runway.
Police said there was a lot of road damage around the city, mostly towards the west end.
Damage was reported from Taitapu Hotel, and there were reports of possible minor injuries.
Roger Sutton of Orion Energy told ONE News that almost the whole of rural Canterbury is without power.
He said that power was expected to remain out until around 6pm.
Half of the Christchurch central business district was without power and most of northern Christchurch, he said.
Power was shut down so that the safety of the network could be assessed.
No tsunami threat
The Pacific Tsunami Warning Centre said no destructive widespread tsunami threat existed based on historical earthquake and tsunami data.
But earthquakes of this size sometimes generated local tsunamis that could be destructive along coasts located within 100km of the earthquake epicentre, it said.