The state of emergency in the wake of Saturday's 7.0-magnitude quake had been due to expire on Wednesday at midday (0000GMT).
The aftershock struck at 7:49 am (1949 GMT Tuesday), sending frightened residents rushing into the streets, cutting already fragile power supplies and bringing down loose material from damaged buildings.
Authorities closed a 1.9-kilometre (1.2-mile) tunnel linking Christchurch to the nearby port of Lyttleton after cracks appeared.
The aftershock, the latest of more than 100 that have rocked Christchurch since the main quake, was just six kilometres deep and much closer to the city centre than previous tremors.
Nobody was killed in Saturday's quake, although many residents reported close shaves. The Canterbury health service said some minor cuts and bruises were reported after Wednesday's aftershock but nothing serious.
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The aftershock struck at 7:49 am (1949 GMT Tuesday), sending frightened residents rushing into the streets, cutting already fragile power supplies and bringing down loose material from damaged buildings.
Authorities closed a 1.9-kilometre (1.2-mile) tunnel linking Christchurch to the nearby port of Lyttleton after cracks appeared.
The aftershock, the latest of more than 100 that have rocked Christchurch since the main quake, was just six kilometres deep and much closer to the city centre than previous tremors.
Nobody was killed in Saturday's quake, although many residents reported close shaves. The Canterbury health service said some minor cuts and bruises were reported after Wednesday's aftershock but nothing serious.
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