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WELLINGTON, Dec 20 (Reuters) - A strong earthquake rattled the east coast of New Zealand on Thursday, causing some power outages and minor damage but no casualties.

The Institute of Geological and Nuclear Sciences said the tremor, measuring 6.8 on the Richter Scale, struck at 8:55 p.m. (0755 GMT).

The institute said it was centred about 50 km (30 miles) off the coast, south east of the provincial city of Gisborne on the country's North Island.

Authorities said Gisborne city had borne the brunt of the quake, but damage appeared to be largely superficial.

"There's been damage to power, gas and water supplies and there's been intermittent phone contact," Vince Cholewa, national civil defence, told Reuters.

"We've been told that three buildings have collapsed, but it's been confirmed there was nobody in them and there are no reports of any injuries there," he said.

Most services were now working and a few small fires caused by ruptured gas mains had been put out.

The quake, described as a series of sharp shocks with a roaring sound, was felt widely along the east coast of both the North and South Islands, including in the capital Wellington, which is around 400 km south of Gisborne.

Murray McPhail, who lives about 10 km from Gisborne, said he could see waves in his swimming pool as the quake shook.

"You could just about surf on it," McPhail told the NZ Press Association. "Stuff came out of cupboards, bottles fell off walls, ornaments fell."

A seismologist said the depth of the quake had limited damage and minimised any chance of a tsunami.

New Zealand scientists record around 14,000 earthquakes a year, of which around 20 top 5.0 on the Richter scale.

The last fatal earthquake in the geologically active country, caught between the Pacific and Indo-Australian tectonic plates, was in 1968 when an earthquake measuring 7.1 on the Richter scale killed three people on the South Island's west coast. (Reporting by Gyles Beckford; Editing by Alex Richardson)

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