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A building in central Christchurch is on a lean after a 5.3 magnitude aftershock.
The Strategy building in Victoria Street tilted during the 5.49pm tremor tonight and several nearby roads had been closed to the public, Civil Defence said.
The aftershock was centred within 5km of Lyttelton, 12km southeast of central Christchurch, at a depth of 11km, GNS reported. It was felt strongly in Christchurch.
A spokeswoman for energy company Orion said about 20,000 homes and business had lost power, mostly in the city's southern suburbs.
Electricity had been restored in some areas, with the rest of the network expected to be up and running by 8pm, she said.
Some traffic lights were also out.
Civil Defence said rocks had fallen in Sumner and Redcliffs and there had been reports of liquefaction and flooding in eastern suburbs.
It advised people to check on their neighbours and to be wary of falling debris and dark areas on the road.
Police said they were conducting "reassurance and response patrols" in Lyttelton, Cass Bay and Port Hills.
Christchurch remains devastated following a magnitude 6.3 quake on February 22, which claimed an estimated 181 lives.
That quake followed a magnitude 7.1 quake last September 4.