A sharp shallow aftershock has occurred in Canterbury.
A Radio New Zealand reporter in Christchurch describes the tremor as the most significant since Saturday's initial quake of magnitude 7.1.
Aftershocks have regularly shaken the area since then and GNS Science had been warning that residents should prepare for at least one more strong aftershock.
GNS Science says it was magnitude 5.1 at 7.49am on Wednesday, at a depth of 6km, 10km north-west of Diamond Harbour.
GNS says the aftershock was felt so strongly because it was closer to the city.
Geologist Mark Quigley from the University of Canterbury says it may have been caused by another previously unknown fault. He says the initial earthquake may be putting pressure on adjacent faults.
Another earthquake, magnitude 4.1, occurred earlier at 7.42am, at a depth of 5km, 20km south-east of Darfield.
Civil Defence says any houses not damaged in Saturday's earthquake will have withstood the latest aftershock.
Police have not yet had reports of extra damage. Superintendent Dave Cliff told Morning Report the shock was a reminder of why people need to stay away from the central city and damaged buildings.
All outpatient clinic appointments at Christchurch hospital have been cancelled. Engineers were earlier checking the buildings for damage.
The hospital is on back-up generator power supply and at this stage no services have been disrupted.
Christchurch international airport remains open.
Mayor Bob Parker told Nine to Noon it is frustrating that when you think you are making progress, you have to go back and review the situation.
Earlier tremors
Two other shocks of magnitude 3.5 and 3.7 occurred at 6.22am and 6.09am respectively. They were both 20km east of Darfield, 15km and 8km deep respectively.
A 4.6 earthquake was recorded at 12.41am on Wednesday. It was centred 30km south-east of Darfield, at a depth of 12km.
A 4.5 one was recorded at 3.59am. It was within 5km of Darfield at a depth of 9km. Another, of magnitude 3.7, was recorded at 3.47am, 30km south-west of Kaiapoi at a depth of 15km.
Earlier, a small cluster of tremors occurred around 9pm on Tuesday.
A series of strong quakes also occurred around Darfield on Monday night, with magnitudes of 3.9 at 9.08pm, 3.8 at 9.07pm, 3.9 at 8.52pm, 3.5 at 8.58pm and 3.5 at 8.08pm.
The series began with a magnitude 7.1 earthquake at 4.35am on Saturday. A state of emergency is still in force.
