Buildings have collapsed and vehicles buried beneath debris, with rescue workers scrambling to help those trapped under rubble left by the 6.3 magnitude quake.
The fire service said the proximity of the quake to the city's centre meant it had caused more damage than the 7.1 magnitude tremor in Christchurch last September.
"The shake has been a lot worse, maybe not in intensity but as far as damage is concerned, and there are numerous people trapped," a spokesman said.
It said: "Other reports include multiple building collapses, fires in buildings in the central (city) and persons reported trapped in buildings."
Those killed are thought to include the occupants of two buses that were crushed by falling debris.