|The magnitude 7 quake caused extensive damage to infrastructure across the city centre [EPA]
New Zealand has extended a state of emergency in Christchurch as aftershocks continued to rock the second-largest city following a major earthquake two days ago.
Security forces moved into the city centre on Monday after the magnitude 7.1 quake smashes buildings and homes, wrecked roads and rail lines, but caused no deaths.
Mayor Bob Parker extended a state of emergency for another two days as troops moved to help police secure streets and badly-damaged businesses in the worst-hit central area of the city.
Parker said services were being restored after the quake cut power across the region, blocked roads with debris, and disrupted gas and water supplies.
Police and troops sealed off the city centre, with the Civil Defence authority saying the area will be cordoned off due to danger from falling masonry and glass, with only building and business owners allowed access.
"We'll be posted on the perimeter of the area of restricted access around the CBD [central business district] and making sure we restrict movement to only people who need to be in CBD," Major Craig Wilson said.
"[This will free up] the police staff to conduct their work around us and inside the CBD to ensure things are kept orderly and secure."
"If this had happened five hours earlier or five hours later [when many more people were in the city], there would have been absolute carnage in terms of human life"
John Key, New Zealand prime minister
Schools, shops and businesses were closed and residents were advised to stay at home until inspections of the more than 500 buildings damaged in the country's most destructive quake for almost 80 years were completed.
The extent of damage is estimated at $1.44bn.
John Key, the New Zealand prime minister, said it was a miracle that no one was killed.
He said there were no major injuries reported partly because the quake happened before dawn.
"If this had happened five hours earlier or five hours later [when many more people were in the city], there would have been absolute carnage in terms of human life," he told TV One News.
The cabinet is scheduled to meet on Monday to make initial decisions on assistance for the Christchurch region.
New Zealand sits above an area where two tectonic plates collide.
The country records more than 14,000 earthquakes a year, but only about 150 are felt by residents. Fewer than 10 a year do any damage.
New Zealand's last major earthquake registered magnitude 7.8 and hit South Island's Fiordland region on July 16, 2009, moving the southern tip of the country 30 centimetres closer to Australia.