The first 5.8 magnitude quake struck at 1358 local time (0058 GMT), the US Geological Survey said. Another of similar magnitude hit 80 minutes later.
Dozens of people suffered minor injuries but only 19 were admitted to hospital.
It comes 10 months after swathes of the city were destroyed by another quake.
Friday's first earthquake struck 26km (16 miles) north-east of the city at a depth of 4.7km. The second large event, with a magnitude estimated at 5.9, was in almost exactly the same place, the USGS said.
Aftershocks rolled on throughout the afternoon, several of them with a magnitude greater than 5, according to New Zealand's GeoNet.
Buildings were damaged, power supplies cut and the city's airport had to be closed temporarily.
There were rockfalls in some coastal suburbs of the city and the authorities have warned people to stay away from hillside areas.
About 26,000 residents were without power at one point, but by 2200 local time the supply had been restored to all but about 400 customers in the New Brighton area, power company Orion said in a statement.
On its twitter feed, the supplier advised customers in the eastern suburbs to be prepared for intermittent power cuts over the next few days.
Canterbury Civil Defence said there was no need for residents to boil water as the city's water infrastructure was fine.
Sink holes and flooding have been caused by liquefaction in some suburbs
Some shopping centres were evacuated and several supermarkets remained closed on one of the busiest days of the year.
Christchurch Mayor Bob Parker said he was delighted that most of the city's shopping precincts intended to open on Saturday.
"It is very important we bounce back from today's events. For retailers it is one of the biggest trading days of the year and they need to open."
Residents can check for updates on shops and infrastructure on Christchurch City Council's twitter feed and on the earthquake information site run by Environment Canterbury.
'Not again'
Mr Parker said events had left people shaken.
"Psychologically this is a tough one. People are outside buildings on streets weeping, I'm told."
"You can just sense the sense of 'not again, how much longer'," he told the Newstalk ZB radio network.
New Zealand's Prime Minister John Key said he was being kept informed as information from the affected areas comes in. "My heart goes out to the people of Christchurch and Canterbury at this time," he said.
Big potholes have appeared in the roads in the Bexley district.
A Radio New Zealand reporter, Bridget Mills, said in places the tarmac felt "like jelly", and there was fresh flooding caused by liquefaction.
Liquefaction occurs when the soil and sediments lose their strength and stiffness in response to the changes in pressure caused by an earthquake, and temporarily behave like a liquid.
The phenomenon has also caused flooding in the suburb of Parklands.
The earthquake in February killed 181 people and caused up to NZ$20bn ($15.5bn, £9.9bn) in economic losses.
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