The New Zealand government geological agency GNS Sciences said the quake struck at 1.58 p.m., 20 km northeast of Lyttelton and 8 km deep.
The quake, which lasted for about 20 seconds, was followed by aftershocks.
Some household items were broken and many people ran out of buildings. Christchurch airport, supermarkets and shopping malls have been evacuated.
Wang Xinmin, China's deputy consul general to Christchurch, told Xinhua that he was driving when the quake struck and he was forced to pull off the road because of the shaking.
Some phone networks were down and power is out in Christchurch suburb of New Brighton.
A police spokeswoman said she did not know whether there were any reports of damage or injuries so far, but added the phones were really busy. A 6.3 magnitude quake rocked Christchurch Feb 22, killing 165 people.
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