Police say a number of streets in central Christchurch will remain cordoned off until tomorrow morning.
Shallow and violent shocks were felt overnight and during the day today, with the largest a 4.9 magnitude jolt at 10.30am.
The aftershocks come nearly four months after the area's 7.1 magnitude quake on September 4.
Fire Service Christchurch area commander Dan Coward said five teams of firefighters, council engineers and Urban Search and Rescue (USAR) workers were this evening assessing central city buildings for damage.
The teams had identified 20 buildings with possible problems, including the Guthrey Centre in Cashel St.
They were using a building "sticker" system similar to the one used after the September earthquake, and would continue to make assessments into the evening.
Christchurch police central area commander Inspector Derek Erasmus said there had been several cordon changes as a result of new building assessments.
Earsmus said parts of Cashel St, Gloucester St, Manchester St and Tramway Lane would remain cordoned off overnight.
A number of individual buildings which have suffered damage will remain fenced off.
Overnight cordons would help ensure public safety until further building assessments can be carried out tomorrow,
"We need to maintain cordons in these locations because of the potential risk to safety from damaged buildings, including falling masonry and brickwork, and damaged windows," he said.
"These cordons will be assessed again tomorrow and we hope that we can further reduce the extent of the road closures.
"We want to cause the least disruption possible so that activity in the city can return to normal."
Police staff and dog handlers would be patrolling the central city overnight to maintain the cordons and to monitor building security, he said.
Officials were contacting the owners of any buildings identified as "high-risk" to speak about the options and any necessary repairs.
Twenty buildings had been identified by the teams, but Coward said he did not want to name them until the owners had been spoken to.
Acting Mayor Ngaire Button said the emergency centre which had been set up to co-ordinate officials' work would remain open until the building assessments were completed.
Button said there had been no reported damage to the city's infrastructure, and power had been restored in all areas.
Button said she was staying in contact with Mayor Bob Parker, who is on holiday, to brief him on events and get any necessary information.
Power was out in parts of the CBD after the 10.30am aftershock, which was magnitude 4.9 and located within 5km of Christchurch.
Button said the council had called back in key communications and civil defence staff to get "all the information we can''.
The aftershock cut power to an estimated 40,000 Orion customers.
Orion chief executive Roger Sutton said most of the outages were reported in the central business district although there were also cases in Papanui, St Albans, Fendalton and other parts of Christchurch.
Almost all of the power was back on within 30 minutes and a small base of customers had to wait one hour.
The Orion network had not suffered any significant damage.
"As a precaution we have cordoned off two blocks bounded by Oxford Tce, Lichfield St, Hereford St, and Colombo St until building inspectors have had an opportunity to assess the damage," Erasmus said.
Fire brigade crews were called to some "life-risk'' incidents following today's aftershocks but there have been no reports of injuries.
Officials said the damage was mainly localised to the central business district and was not nearly as severe as the September 4 earthquake.
Coward said the Fire Service had fielded more than 200 calls since the first aftershocks at 2am.
Most were alarm activations but Coward said some involved people being trapped in lifts and stairwells.
Erasmus could not say when assessments would be complete to allow cordons to be removed.
He said the aftershock was not as big as previous earthquakes but because it was located in the city centre during daylight hours: "it would have felt very strong to many people.''
Button said the Council had set up its Civil Defence team at the council building which was undamaged after the latest shakes.
She was aware central city businesses were already struggling and were relying on the Boxing Day shopping trade to boost their coffers but public safety took precedent.
"I'm a shopper too but I wouldn't want to get killed,'' she said.
Meanwhile, a Canterbury District Health Board spokeswoman said people with non-urgent injuries were causing problems with waiting lines at the hospital's emergency department.
Anybody with a non-urgent injury should contact their GP or visit an after-hours doctor, she said.
BOXING DAY SHOPPERS SHAKEN
Hundreds of Boxing Day shoppers walked out of one the city's largest malls, Westfield Mall in Riccarton, with many in tears. The mall remained open throughout the day.
In central Christchurch people poured onto the streets as soon as the earthquake hit.
Engineers will inspect Ballantyne's department store after it was evacuated.
Shoppers queued to get into the iconic store to pick-up sales bargains when it opened at 10am.
However, they had less than an hour's shopping time before the big shake hit.
Sergeant Franco Lovrich said there had been some damage to the building and there was a lot of broken glass inside.
Cashel Mall from Colombo St to the Bridge of Remembrance was being closed off as a precaution.
Managing director Richard Ballantyne said everyone had left the store in a "very orderly'' manner.
"There was a loss of power and a good shaking, but luckily the customers and staff stayed very calm and orderly,'' he said.
"There's nothing structural at the moment and we are getting an engineers' report. We will wait for the police and fire service and the report before we make any decisions.''
It was a disappointing blow on a busy trading day, and after Ballantyne's avoided damage in the September quake.
"There's no doubt trade results get very badly affected when we get something like this on a busy day."
Miraculously, no-one was hurt when a large chunk of masonry fell from the building housing the Southern Encounter Aquarium, in Cathedral Square.
American tourist Erik Davidsen said he was standing within a few metres of the building when the red bricks fell down and broke across the paving, smashing part of an awning on the way.
The material did not come down immediately, he said.
"It was actually about a minute later. There was a small shock and a very large piece came down,'' said Davidsen, 52.
The area had already been cleared as people walked into the middle of the square following the main quake.
"It had a short duration and made a very wobbly earthquake. No-one was in the area. I would say people were out right around the perimeter of where the debris is,'' Davidsen said.
Across the square tourist Deborah Frey had been evacuated from the Heritage Hotel.
Frey was in the lift when the shake hit.
"It was very frightening. The elevator felt like it was going to drop,'' she said.
"Everyone was directed to the stairwell and told to assemble (outside).''
Frey was picking up a campervan today to begin a tour of the rest of the South Island.
The continuous aftershocks over the last 12 hours had left her nervous, she said.
"I have to say I am glad to be leaving today.''
Sinead Boucher, a visitor from Wellington and Fairfax digital editor, was in Westfield mall when the aftershock hit. She said the quake caused things to fall from shelves and a collective shriek to go up from the crowd.
Shops immediately closed and people were streaming from the mall, she said.
"I was almost knocked on the head by a large shop sign falling off."
The Press building in Cathedral Square has been evacuated, with evidence of fallen plaster and external cracks to the 100-year-old building.
Earlier today, a Christchurch resident tweeted: "I had four broken vases in the original 7.1. That one broke at least another six and books shelves down. House a mess."
Masonry has been reported lying on the corner of St Asaph and Madras streets.
St John Ambulance this morning reported a "very slight'' increase in the number of calls after the 10.30am aftershock. However it said there had been no reports of any injuries from any of the aftershocks.
Christchurch had not experienced a significant aftershock for several weeks; the last one over magnitude 4 was a 4.4 jolt on November 27.
Opawa resident Mandy Walker said she was jolted awake just after 2am.
"It woke me up and my boy, he's five, he screamed out to me."
She found the experience frightening.
"It was just the noise - everything sort of rattled and I was wondering, 'Is it another big one?'"
Shortly after the 2am aftershock felt by Christchurch residents, a 7.2 magnitude earthquake struck near Vanuatu at a depth of 33km.
Both New Zealand and Australia's emergency warning systems issued advice that no tsunami was expected.
The earthquake was centred 135 kilometres west of Isangel on the Island of Tanna in Vanuatu. Tsunami alerts for Fiji, Vanuatu and New Caledonia were withdrawn about 4am.
Meanwhile, a 5.9 magnitude earthquake centred at White Island was widely felt in Gisborne this morning.
Reisdent Barry Cook said he was shaken awake and immediately waiting for an aftershock.
"It was a goody,'' he said.
"One of the biggest I've felt since our one here a couple of years ago that caused all the damage.''
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