Christchurch has been rocked by its largest aftershock in almost four weeks.



The magnitude 5.0 aftershock struck at 10.21pm on Monday night, at a depth of 12km, 10km west of Christchurch. A second quake, measuring 4.0, hit at 10.29pm.



Both quakes were felt throughout the Canterbury region.

They were followed by a 4.4 shock which struck 20km south of Darfield at 6.29 this morning, which was felt in Christchurch.



The first aftershock was the region's largest since a 5.1 magnitude tremor that caused further damage to the city on September 8, following the large 7.1 quake which hit the city a month ago.



GNS Science seismologist John Ristau said the 5.0 quake was "quite shallow" and "potentially damaging" to property.



"We haven't seen anything that size since the first few days [after September 4]," he said.



"If there were structures that were already significantly weakened from the main shock, this could cause a lot more damage than you might normally expect for a magnitude five."



Police southern communications spokesman John Doherty said there had been no immediate reports of damage.



Canterbury has now had more than 1000 aftershocks since the initial 7.1 quake.



Ristau said that while the rate of aftershocks had slowed, the activity could continue "for months".



"There is always the potential still for the odd large one to come through," he said.



