There are reports of widespread destruction in the city, whichw as hit by a quake last September.
Radio New Zealand's reporter says staff at the Christchurch newsroom had to cling to their desks during the shaking. Large filing cabinets toppled.
People made their way out of the building but when they got out they couldn't see for dust.
The quake was centred at Lyttelton at a depth of 5km.
Phone lines are down and calls are not being connected to emergency services, The New Zealand Herald reports.
The quake was felt as far away as Wellington and Dunedin.
Our Kiwi friends do not deserve this. I pray and hope no lives have been taken.
Are there any cruise ships at Lyttelton today?