GNS seismologist Brian Ferris said people would have felt about 150 of those aftershocks.
About 10 aftershocks were between magnitude 5 and 5.5, 60 between 4 and 4.9 and 200 between 3 and 3.9, he said.
Since midnight there had been 18 aftershocks.
A 5.1 magnitude aftershock that struck shortly before 8am today caused the most alarm in Christchurch since the initial quake, as it was a relatively shallow 6km deep and centred around the city's port area of Lyttelton.
ODT reporter Debbie Porteous said the "short, sharp shock" cut the power in her central city hotel room and knocked over glasses and cups.
"We ran to the doorway to shelter," she said.
The aftershock caused more damage to already weakened buildings and forced another evacuation of the central city.
Christchurch Mayor Bob Parker said the gut-wrenching intensity of the latest quake 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 NewstaklkZB. "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.
"Oh shit, excuse me. Holy heck... sorry about that," he said.
Mr Parker said assessment teams were heading back into the city to check the damage but emergency workers and civil defence workers were now extremely worried about their own families.
"We are just so jumpy . It doesn't matter how calm you be in these issues, there is still a part of you which wants to tear out the door and head to the hills but that passes quickly," he said.
Mr Parker said hearing the earthquake coming was "like an incoming doodlebug".
Doodlebugs were the flying bombs launched by Nazi Germany at England during World War 2
"There is no way you can get out of its way. You just stand there and you hope," Mr Parker said.
GNS Science warned that the aftershocks may continue for some time.
Tony Stuart, a roofing contractor who lives on the Cashmere Hills, said he was in his office at his home when the latest earthquake hit.
"There was stuff off shelves, the other part of my neighbour's wall has fallen off," he said.
"This is the biggest aftershock we have had. There is stuff falling all over the place.
"It is very scary.
"Water has even splashed out of George's bowl (his fox terrier dog)," Mr Stuart said.
Opawa resident David Alexander said it was a "helluva shake", and his family had dived under the table for protection.
"We've got more stuff down, we almost had the house back in order."
Mr Alexander, lives in a villa on loamy soil, lost one of three chimneys to the initial quake on Saturday.
The other two were cracked from the ceiling to the floor and he feared today's shake would topple tonnes of bricks into his home.
"We thought it was going to come down this time."
His next-door neighbours have abandoned their house and gone to a welfare centre, they are so frightened.
Mr Key, who cancelled his trip to Europe yesterday, is currently on his second trip to Christchurch following Saturday's quake and this morning visited rural areas including Kaiapoi, where volunteers are helping locals clean up silt on properties and roads.
"At this stage it would be my intention to come back on Saturday, although that's very much a work in progress at the moment, and then I'll return next week," he said.
He visited welfare centres in the city last night, digging in to help serve up food at Addington racecourse. The numbers in the centres rose yesterday after three overnight aftershocks caused more alarm and damage.
Mr Key said the duration of the current state of emergency was up to the local mayors and civil defence.
"It should last as long as it makes sense. Everyone is keen to restore the city and the surrounding districts back into a fully operational capacity as soon as possible, but we are dealing with some very difficult situations.
"Of paramount importance is the safety and security of Christchurch residents and surrounding areas.
"As this disaster unfolds what we're seeing is some areas are much more badly affected than we thought they were and, in fact, the damage is much greater than we thought it was."
