New Zealand Prime Minister John Key Wednesday vowed to rebuild Christchurch, telling its residents the quake-hit city would rise again despite suffering a "ruthless act of nature".

Key said that every New Zealander had a connection to the southern city, which was flattened Tuesday by a 6.3-magnitude tremor that caused buildings to tumble, crushing at least 75 people to death.

"Today all New Zealanders grieve for you, Christchurch," he said.

"On behalf of New Zealand let me say to all of you: we feel your pain, as only a small nation can, for none of us feel removed from this event."

Key, who was raised in the South Island city, described himself as "proud to be a son of Christchurch".

The deadly quake is the second major tremor to hit Christchurch in the past six months, but the 7.0-magnitude quake that hit in September caused no deaths.

Tuesday's tremor hit the centre of the city hard, toppling the spire of its landmark cathedral, reducing several buildings to rubble and filling the streets with broken masonry.

The prime minister said he had been cheered by the support he had received from around the world in the wake of the quake, as he urged the city's residents: "You will get through this."

"Christchurch, today is the day your great comeback begins," he said. "Though your buildings are broken, your streets awash, and your hearts are aching, your spirit will overcome."

As he pledged his government to doing everything it could to support the recovery and rebuilding process, Key said the city was on a journey from "ruins and despair to hope and new opportunities".

Key said many cities in the world had come back from disasters and Christchurch would be no exception.

"We will rebuild this city resolutely," he said.

"Though lives lost will never be replaced, though your city will never look the same again, you will rebuild your city, you will rebuild your lives, you will overcome."

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