The U.S. Geological Survey said the quake struck just before 1 p.m. Tuesday (7 p.m. Monday ET), and a 5.6-magnitude aftershock struck about 15 minutes later.
The New Zealand Herald reported that phone lines in the area were out, roads were cracked -- in some cases lifted as much as a meter -- and water mains had burst, flooding several streets.
Christchurch police told TVNZ that the city's 106-year-old Cathedral of the Blessed Sacrament was badly damaged, and a Herald reporter said that half of the building had collapsed.
TVNZ reported that the 147-year-old Christchurch Cathedral's spire had toppled, Christchurch Hospital was being evacuated and the airport was closed.
The quake caused significant damage to several older buildings, the Herald reporter said.
Christchurch Mayor Bob Parker told Radio New Zealand that the rumbling tossed him across the room, that he knew of injuries in the city council building and has heard unconfirmed reports of serious injuries.
"That was, in the city central anyway, as violent as the one that happened on the 4th of September," he said.
Parker added that streets were jammed as people tried to get out of the city, and he urged people to avoid the water supply.
"We've been through this before this once, we now need to think what we did at that time," he said.
Southern New Zealand has been plagued by a series of quakes since September, when the area was shaken by a 7.1-magnitude temblor that New Zealand authorities said was the most damaging quake to hit the region since 1931. The earthquake struck in the predawn hours of September 4, and authorities said the deserted streets at that time likely kept injuries to a minimum.