(AFP) - A powerful 7.8-magnitude earthquake killed two people and caused massive infrastructure damage in New Zealand on Monday, as forecasters warned wild weather could hamper rescue efforts.

The tremor, one of the most powerful ever recorded in the quake-prone South Pacific nation, hit just after midnight near the South Island seaside tourist town of Kaikoura.

It triggered a tsunami alert that sent thousands of people fleeing for higher ground across large parts of the country's rugged coastline before the threat abated.

Rescuers were left scrambling to reach Kaikoura, which had no telecommunications and was isolated by landslips, making it accessible only by helicopter.

Civil Defence Minister Gerry Brownlee said a clearer picture of the scale of the damage was slowly emerging.

"I think had there been serious injury or suspected further loss of life than we would have heard about it by now," he told Radio New Zealand.

He added: "It looks as though it's the infrastructure that's the biggest problem, although I don't want to take away from the suffering... and terrible fright so many people have had."

Brownlee and Prime Minister John Key flew over the affected area in a military helicopter.

Aerial footage outside Kaikoura showed railway tracks ripped up and tossed 10 metres (30 foot) by the force of the quake.

Landslips dumped hundreds of tonnes of rocky debris on the main highway while locals posted pictures of themselves near huge fissures that had opened up in roads.

One person was believed to have died at a historic homestead which collapsed at the town, while police were trying to reach the scene of a fatality at a remote property north of Christchurch.

The earthquake struck at 12:02am Monday (1102 GMT Sunday) and was 23 kilometres deep, the US Geological Survey said, putting the epicentre in the South Island's North Canterbury region.

It was felt across most of the country, causing severe shaking in the capital Wellington, about 250 kilometres (155 miles) away.

The quake ignited painful memories for residents in nearby Christchurch, which was devastated five years ago by a 6.3 tremor which killed 185 people.

Key said he was well aware its potential impact could have been much worse.

"Purely on the Richter scale, this thing has been bigger than what we saw in the Christchurch quake, but thankfully the loss of life, at this point, is significantly less," he told Sky News.

- Terrifying experience -

Earthquake engineer Ken Elwood from the University of Auckland attributed the relative lack of fatalities to the quake's midnight timing and the fact it hit a sparsely populated rural area.

In contrast, the Christchurch quake hit at lunchtime directly under one of the country's biggest cities.

"When it happens in the middle of the day it's a very different story," Elwood told TVNZ.

"People were safe in their homes, homes might get damaged but they're safer for the people inside and that's certainly the blessing of this earthquake."

But for residents living through the tremor, particularly those who survived the Christchurch disaster, it was a terrifying experience.

"It was massive and really long," Tamsin Edensor, a mother of two in Christchurch, told AFP, describing it quake as the biggest since 2011.

"We were asleep and woken to the house shaking, it kept going and going and felt like it was going to build up."

Soon after the earthquake, tsunami warning sirens were activated, with police and emergency workers going door to door to evacuate seaside properties.

Civil Defence, responsible for emergency management in New Zealand, warned of a "destructive tsunami" with waves of up to five metres (16 feet).

However, the largest waves were only about two metres before the alert was lifted.

Hundreds of aftershocks, some stronger than 6.0, continued to rattle the country in the hours after the main quake.

Among those given a scare were the touring Pakistan cricket team, who were staying in Nelson, about 200 kilometres from the quake centre.

"Some of the boys were in prayer, some were watching the India-England Test on TV when we felt the windows shake," team manager Wasim Bari told ESPNcricinfo.

Conditions still remained treacherous late Monday with officials urging evacuations after debris blocked the South Island's Clarence River, then shifted to release a "wall of water" downstream.

Forecasters predicted wild weather, including strong winds, overnight, hampering rescue efforts.

New Zealand is on the boundary of the Australian and Pacific tectonic plates, which form part of the so-called "Ring of Fire", and experiences up to 15,000 tremors a year.

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