A little over an hour later, at 3.18pm, Christchurch residents were rocked by a quake of magnitude 6.0, centred 10km east of the garden city at a focal depth of six kilometres.
More than 600 people reported feeling the quake from Riverton in Southland to New Plymouth.
At 4.50pm another quake struck, this time measuring 5.0. It was 10km deep and was centred 20km north-east of Lyttelton.
At 5.08pm the area was hit by a quake measuring 4.0, followed by a 3.7 tremor at 5.33pm.
GNS duty seismologist Ken Gledhill said the quakes should taper off over the next week.
He said the latest tremors are almost a repeat of what happened in June with magnitude five and six quakes.
Mr Gledhill was asked if the latest round of quakes were aftershocks from the September 2010 quake.
"If September 2010 hadn't have happened, these would not have happened, or at least not now, but maybe in a 1000 years.''
Minor injuries only
St John Ambulance communications has received up to 150 emergency calls since the first earthquake this afternoon.
It said paramedics have assessed and treated around 50 to 60 patients with earthquake-related injuries or issues.
All of these were minor injuries or issues, and included falls, anxiety issues, panic and stress issues.
St John says it has been busy, but coping, with 14 ambulances and rapid response vehicles working in Christchurch.
After the initial earthquake, Christchurch Airport was evacuated and flights cancelled or diverted, but just after 5:30pm it re-opened.
Christchurch Airport chief executive Jim Boult says there has been no damage to the airport's runways, which are now open for landings and departures.
Mayor Bob Parker says nothing can prepare Cantabrians for more earthquakes.
Mr Parker is urging people to stay off the streets to keep them clear in case of emergency.
"Psychologically this is a tough one. People are outside buildings on streets weeping, I'm told. You can just sense the sense of `not again, how much longer?'' he told Newstalk ZB.
All available police units have been mobilised across the city to provide reassurance and check on safety and damage.
There will be no mail deliveries in the Christchurch area by New Zealand Post tomorrow.
The Christchurch Mail Centre has been evacuated as a precaution for the safety of staff and will not re-open for processing this evening.
The next scheduled New Zealand Post mail deliveries following the statutory holidays will be 28 December.
Post shops will be open tomorrow on a case-by-case basis at the discretion of individual store managers.
Earthquake Recovery Minister, Gerry Brownlee, is travelling back to Christchurch this afternoon to assess the situation.
Hospitals coping well
David Coetzee, National Civil Defence controller, said Christchurch hospitals were coping well and local authorities and emergency services did not expect to declare a state of emergency or require help from other centres.
There were reports of 19 injured people, but all were relatively minor.
"They are large earthquakes, so we can expect damage. This is significant. But the effect from an injuries perspective could have been much worse. I think we are lucky that we had the CBD already cordoned off so there weren't people in there.''
He said he had heard one building inside the cordon CBD had collapsed, although that was yet to be confirmed.
"It could have been far, far more serious if that area was not vacated.''
The city council was sending teams into the suburbs to check on communities and some people were leaving their houses for the night because of safety concerns.
"There are reports of further structural damage to houses as well as some business premises but the extent is not significant, and they do not believe they need to open welfare centres for the night. However, they will keep their finger on the pulse and will do so if need be.
"It is reasonable to expect there will be further degradation of some already damaged structures. Those will be assessed in time. It is too early to comment on the extent of that.''
He urged residents to call the council or 111 if they needed assistance. The council would run its emergency operation centre overnight to be ready to respond to any needs and assess damage.
"These earthquakes were significant. There was significant ground energy and movement again, very similar to the June event. So there are reports of new liquefaction and some structural effects.''
He said rockfalls and further aftershocks were to be expected. There were significant power outages in the eastern suburbs. While water appeared to be unaffected, council and health officials were checking for any contamination and would issue warnings to boil water if required.
There were minor issues with sewerage which, while not signficant, could affect water supply.
"Either take the precautionary route and boil water, but look out for the official message from the district health board or council.''
Politicians send support messages
Labour and the Greens have both sent messages of support to Canterbury residents following today's earthquakes.
Labour leader David Shearer said he joined with the rest of the country in sending heartfelt thoughts to the people of Christchurch.
"Cantabrians have suffered so much since the first earthquake in September last year. It must be truly heartbreaking for them,'' he said.
"Emotions will obviously be raw. I know words really can't do justice to the traumatic situation these people find themselves in.''
Green Party co-leader Russel Norman also had sympathetic words, and said his party was committed to doing all it could to ensure those affected receive the support they need.
"Today's earthquakes are an unwelcome intrusion on the Christmas period when families are coming together after such a challenging year,'' Dr Norman said.
"The people of Christchurch have shown great resilience in the face of adversity, and the human spirit in the people of Canterbury is indomitable.
"We have seen this human spirit in the past, and I have no doubt we will see it over the coming months.''
Earlier, Prime Minister John Key expressed his sympathy, saying Cantabrians would be frightened and disheartened by the latest quake, and his heart went out to them.
"However, residents can be confident that the authorities are onto the situation and government resources stand ready to assist wherever they are needed.''
Mr Key said he was being kept up to date with the situation.
"The Government's resolve to work with the people of Christchurch and Canterbury to rebuild remains unchanged following today's two aftershocks,'' Mr Key said.
