Aftershock hits Christchurch

Christchurch Mayor Bob Parker said the gut-wrenching intensity of the latest quake - which occurred at a depth of just 6km and was only 10km southeast of Christchurch - reduced many emergency workers to tears and led to the evacuation of the Civil Defence offices.

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Earthquake in Christchurch

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A 5.1-magnitude aftershock has hit Christchurch, causing more damage and another evacuation of the city's central business district.

The powerful temblor, which happened just before 8am (0600 AEST) on Wednesday, was originally described as being a 6.1-magnitude quake, but was soon downgraded.

It was felt as the strongest aftershock in the city since Saturday's 7.1 earthquake, which smashed buildings and homes, wrecked roads and disrupted the central city.

New Zealand Prime Minister John Key was told about the latest quake while on his way to inspect Kaiapoi on Wednesday morning.

Christchurch Mayor Bob Parker said the gut-wrenching intensity of the latest quake - which occurred at a depth of just 6km and was only 10km southeast of Christchurch - 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 are just churning up here. When will this thing end? It is like living in a maelstrom."

He said that on Tuesday, 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 Wednesday's big shake would cause more serious damage.

As he spoke the city was rocked by another shake, which measured 3.8.

"Oh s****, excuse me. Holy heck, sorry about that," he said.

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 to you to 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.

GNS Science warned that the aftershocks may continue for some time and Christchurch City Council said the state of emergency was likely to remain in place, although it would be reassessed during the day.