The latest quake struck 20km south-west of Darfield at 1.41pm and had a focal depth of 5km, according to
GNS Science.
This morning's magnitude 5.1 aftershock did not cause any further substantial damage to Christchurch buildings, initial assessments suggest.
However, Civil Defence authorities said some buildings would need to be reassessed after the quake, which struck at 7.49am.
All 678 buildings in the central business district have been checked with 69 per cent being assessed as safe. Others have been rated safe for limited access and unsafe. In Kaiapoi, 22 buildings have been declared unsafe to enter and some have been demolished.
The state of emergency in New Zealand's second largest city was due to end today but a spokesman for Christchurch City Council said that it will now be extended for another week.
The 10 or so aftershocks this morning have prompted calls from Christchurch Mayor Bob Parker to remain calm.
Mr Parker asked residents to avoid travelling and stay at home with their families if it was safe to do so.
Civil Defence director John Hamilton said the aftershocks were expected to continue for at least the next week.
There had been reports of fresh damage from this morning's big aftershock with power cut to some areas of the central city and security alarms going off as it was restored.
Some traffic lights are out and police have been directing traffic at intersections and manning road blocks.
The Lyttelton Tunnel was closed while it was checked for structural damage but re-opened at 11.30 when only superficial damage was found.
Another big aftershock, measuring 4.4 on the Richter scale, hit near Rolleston at 9.01am, while one measuring 4.0 was centred near Lyttelton Harbour and struck at 9.39am.
'Staff in tears'
Mr Parker said the gut-wrenching intensity of the 5.1 aftershock reduced many emergency workers to tears and led to the evacuation of the Civil Defence offices.
"It was a devastatingly, vicious sharp blow to the city," he told NewstalkZB.
"This was a terrifying moment. We have just had to evacuate our Civil Defence headquarters.
"We have got staff in tears, we have got fire engines going through the middle of the city, power is out and a lot of people are very, very churned up by that.
"My guts is just churning up here. When will this thing end? It is like living in a maelstrom."
He said that yesterday people's confidence was beginning to return as the gaps between aftershocks lengthened.
"We were restarting to think, maybe, just maybe, we are over the worst of this and now we have had this shocking event.
"This is a hammer blow to the spirit of a lot of people."
He said people would recover but "we have got the most amazing city, the most amazing people who have been through four days of physical and mental hell".
He said today's big shake would cause more serious damage.
As he spoke the city was rocked by another shake, which measured 3.8.
Cantabrians were told to listen to their radios for further updates.
Civil Defence has issued a warning that an aftershock of six is possible in the next few days.
Christchurch Hospital has cancelled all outpatient clinic appointments today because it says people are not keen on leaving their homes after the aftershocks.
5.1 aftershock
This morning's 5.1 aftershock was only 6km deep and struck at 7.49am while Prime Minister John Key was speaking to residents in rural Canterbury.
Kaiapoi residents who Mr Key chatted with spoke of a strong jolt but Mr Key himself did not appear to feel the shaking.
One Christchurch resident described the aftershock as "violent" and likened it to a truck hitting her house.
In the CBD, Kristy Richardson said she was on the bus on her way to work at Pak N Save when the vehicle had to pull over.
She said the jolt lasted about 20 seconds.
English tourists Luke Gibbs and Laura Collins said they were standing under a hotel awning when the aftershock struck but shot out from underneath it, afraid it might fall.
They said they were leaving Christchurch today but the earthquake had not affected their plans.
The pair said they had only felt one earthquake before and thought people back home would not believe what the experience was like.
Staff from the Yellow Pages building in the city were forced out when a sprinkler went off following the aftershock.
Many of the staff gathered on traffic islands to avoid any falling debris.
However, some Christchurch residents are becoming used to aftershocks and others have had unexpected benefits from the ongoing earthquakes.
James Minson and Fiona Skipper told nzherald.co.nz they didn't even get out of bed for this morning's 5.1 shallow aftershock.
"Maybe it's just hard-ass South Islanders [not being worried]," Ms Skipper said.
Their friend Naith Morrow said he has been waiting to see how big the quakes were before reacting. "You wait for about 30 seconds to see if its going to get bigger."
But he admitted the tremors had caused him to "sleep downstairs".
Lou and Paulus Van Bussel run their small cafe Escarto from a cart in Cathedral Square. While their competitors around them have been forced to close they can stay open because they have their own water they have been boiling and aren't in a building.
But when the earth moves, they know it.
"It was pretty wild in here, rocking and shaking. Two customers we had leapt back [from the cart," Mrs Van Bussel said.
They only would have had the umbrella fall on them," Mrs Van Bussel said.
Mr Van Bussel told of listening to the windows in offices around them rattling as the buildings shook.
While they might have the "safest cafe" it was "a lot of effort boiling the water".
Twitter reaction
Some Christchurch residents on Twitter reacted to the new aftershock with fear and, at times, humour.
One resident, Moata Tamaira, she had "just moved the booze to within reach of the doorframe. Priorities."
Another pointed out what the rest of New Zealand was thinking as news of the aftershock spread: "That's a whole new quake! Hope everyone is canty is ok!"
Another resident noted: "Apparently that one was near Lyttelton. No wonder it felt so big. Power back on already though. Phone network seems to be down."
Orion power network got a pat on the back for keeping power operating to parts of Christchurch: "Kudos to Orion. Power back at my house already. You guys are awesome. You must be so tired by now and you keep on going. We luff you."
One resident said that the aftershocks had caused more objects to fall than the original earthquake on Saturday: "My parents have more stuff on the floor with that latest earthquake than on Saturday."
There have been over 300 aftershocks since Saturday morning's 7.1 magnitude quake.
In other developments:
* The Queen wished Cantabrians well after Saturday's devastating earthquake.
"The Queen has today asked the New Zealand prime minister to send her good wishes to those affected by and helping with the aftermath of Saturday's earthquake in Christchurch," a Buckingham Palace spokesman said overnight.
* Police said last night that the number of family violence incidents in Christchurch had soared more than 50 per cent since the quake, because of "significant stress". Other offences, including burglary, had fallen.
* The Government last night announced an emergency $350-a-week wage subsidy for businesses with fewer than 20 employees which can't trade or pay staff because of earthquake damage.
It will last for four weeks and businesses will be able to re-apply after that.
* Christchurch schools are closed today but may re-open tomorrow if Boards of Trustees decide that it is safe for students to return to school.
* At least two inner-city heritage buildings will be demolished soon because of fears they will collapse.
- ADDITIONAL REPORTING: NZPA
Updated with lots more information about damaged historic buildings, damage from this morning's aftershock etc. (Now shows damaged properties as far north as Woodend and as far south as Timaru.
Updated with lots more information about damaged historic buildings, damage from this morning's aftershock etc. (Now shows damaged properties as far north as Woodend and as far south as Timaru.
Map: Tracking the aftershocks
KEY
Red markers:
Aftershocks on Saturday
Blue markers:
Aftershocks on Sunday
Yellow markers:
Aftershocks on Monday
Green markers:
Aftershocks on Tuesday
Purple markers:
Aftershocks today
View Christchurch earthquake: Where the land shook in a larger map
