This morning, Christchurch was hit by the most damaging aftershock since the 7.1-magnitude quake on Saturday, forcing evacuations and cracking a major road tunnel.

The aftershock measured 5.0 in magnitude and rattled the Canterbury city at 7.49am Christchurch-time. The tremor sent residents scrambling into the streets, cut power supplies, and exacerbated what is already estimated to be billions of dollars of damage, Telegraph.co.uk reported.

Seismologists said that this latest aftershock ran comparatively shallow at six kilometres deep and much closer to the city’s centre than the main quake on Saturday. It was preceded by a 4.1-magnitude shock and followed by one measuring 3.8.

As a result of this morning’s aftershock, the state of emergency in Christchurch has been extended a further seven days, reported The Sydney Morning Herald.

The extension gives Civil Defence the power to close buildings and restrict access to certain areas.

Infrastructure was yet again disrupted as the aftershock cracked a 1.9-kilometre tunnel linking Christchurch to the nearby port of Lyttleton, the New Zealand Transport Authority (NZTA) said.

"There is some cracking that's apparent on the surface and there's apparently some damage to the canopy as well," NZTA spokesman Andy Knackstedt said.

"We don't know what the extent of that is. They're in there right now making an assessment."

The aftershocks came at a time when the city was starting to mend itself following the destruction that began over the weekend.

"It was a devastatingly, vicious sharp blow to the city," the mayor of Christchurch, Bob Parker said.

"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.

"This is a hammer blow to the spirit of a lot of people."

More than 100 aftershocks have rocked the area since Saturday.

Officials estimate up to 100,000 homes were damaged in Saturday's quake, which caused damage estimated at NZ$2 billion but has not claimed a life.