An rescue operation is underway after strong aftershocks of up to 6.2 in magnitude followed a powerful 7.8 earthquake in central New Zealand early on Monday.
At least two people were killed and thousands of landslides were triggered, cutting off some towns from the rest of the country.
Emergency response teams flew by helicopter to the region at the epicentre of the tremor, which struck just after midnight some 91 km (57 miles) northeast of Christchurch in the South Island, amid reports of injuries and collapsed buildings.
"It's just utter devastation, I just don't know... that's months of work," New Zealand Prime Minister John Key told Civil Defence Minister Gerry Brownlee after flying over the coastal town of Kaikoura, according to Brownlee's Twitter account.
He described landslips in the area as "just horrendous". In a statement seen by Reuters, Key said of the likely damage bill: "You've got to believe it's in the billions of dollars to resolve."
Powerlines and telecommunications were down, with huge cracks in roads, land slips and other damage to infrastructure making it hard to reach the worst-affected areas.
A tsunami warning that led to mass evacuations after the original quake was downgraded after large swells hit New Zealand's capital Wellington, in the North Island, and Christchurch.
Wellington was a virtual ghost town with workers ordered to stay away while the city council assessed the risk to buildings, several of which were damaged by the tremor. Severe weather with 140 km per hour (85 mph), gale-force winds was forecast for the area.
Hundreds of aftershocks, the strongest a 6.2 quake at about 1.45pm local time (0045 GMT), rattled the South Pacific country, fraying nerves in an area where memories of a deadly 2011 quake are still fresh.
Christchurch, the largest city on New Zealand's ruggedly beautiful South Island, is still recovering from the 6.3 quake in 2011 that killed 185 people.
New Zealand's Civil Defence declared a state of emergency for the Kaikoura region, centred on a tourist town about 150 km (90 miles) northeast of Christchurch, soon after Monday's large aftershock.
Kaikoura, a popular spot for whale watching, bore the brunt of the quake. Up to 100,000 landslides were triggered in the region by the shock, the New Zealand Herald reported, some blocking roads and railways, cutting off the Kaikoura from the rest of the country.
"Our immediate priority is ensuring delivery of clean water, food and other essentials to the residents of Kaikoura and the estimated 1,000 tourists in the town," Brownlee said.
The Navy and Air Force have begun evacuating people stranded in the town.
There was at least one death in area, as a large farmhouse collapsed with three people inside. Margaret Edgar, aged 100, was pulled alive from the ruins along with her daughter-in-law, but her son, Louis, was killed.
Nearby, three cows were pictured huddled on an island of grass, the surrounding land having collapsed after the tremors.
Urban Search and Rescue (USAR) said a 20-person rescue team and two sniffer dogs had arrived in the town. A second team was on standby in Christchurch, the USAR said in a statement.
Hours after the quake, officials said a slip dam caused by the quakes that had blocked the Clarence River north of the town had breached, sending a wall of water downstream.
"Residents are urgently advised to move to higher ground immediately," RNZ quoted a statement from the Marlborough District Council as saying.
New Zealand's Geonet measured Monday's first quake at magnitude 7.5, while the U.S. Geological Survey put it at 7.8. The quakes and aftershocks rattled buildings and woke residents across the country, hundreds of kilometres from the epicentre.
Government research unit GNS Science later said the overnight quake appeared to have been two simultaneous quakes which together lasted more than two minutes.
New Zealand lies in the seismically active "Ring of Fire", a 40,000 km arc of volcanoes and oceanic trenches that partly encircles the Pacific Ocean. Around 90 percent of the world's earthquakes occur within this region.
Stock exchange operator NZX Ltd said markets were trading, although many offices in the capital were closed. The New Zealand dollar initially fell to a one-month low before mostly recovering.
Fonterra, the world's biggest dairy exporter, said some its farms were without power and would likely have to dump milk.
Key postponed a trip to Argentina, where he had planned to hold a series of trade meetings ahead of the Asia-Pacific Economic Cooperation (APEC) leaders' summit in Peru this week, as he met disaster officials.
At least one of those killed was found in a house in Kaikoura that "collapsed like a stack of cards", Kaikoura Hospital's Dr Christopher Henry told Fairfax media. Two other people were pulled alive from the same building.
New Zealand Earthquake in pictures
New Zealand Earthquake in pictures
A powerful earthquake struck in a mostly rural area close to the city of Christchurch but appeared to be more strongly felt in the capital, Wellington, more than 200 Km (120 miles) away
Access to the ramp for the Bluebrige Cook Strait Ferry was cracked, after a 7.5 earthquake based around Cheviot in the South island shock the capital, in Wellington, New Zealand
Broken glass and debris covers part of the road on Featherston Street after an earthquake in Wellington, New Zealand
The 7.5 magnitude earthquake struck 20km south-east of Hanmer Springs at 12.02am and triggered tsunami warnings for many coastal areas
New Zealand residents standnear the top of Mt Victoria, the highest hill in Wellington
Ships head out into the harbour in Wellington, following an earthquake centred some 90 kilometres (57 miles) north of New Zealand's South Island city of Christchurch
New Zealand residents park cars near the top of Mt Victoria, the highest hill in Wellington
Amora Hotel guests gather in a carpark after an earthquake in Wellington, New Zealand
New Zealand media reported one of the pilots taking rescuers to the area was Richie McCaw, the recently retired captain of New Zealand's world champion All Blacks rugby team.
"At one point, the railway was way out over the sea - it had been pushed out by (land) slips. It would not have been a nice place to be at midnight last night," McCaw told the New Zealand Herald after helping fly the USAR team to Kaikoura.
Shortly after the quake, residents heeding tsunami warnings in Wellington caused gridlock on the roads to Mount Victoria, a hill with a lookout over the low-lying coastal city.
Around 100 people, including tourists and children sleeping on floors and benches, camped out overnight in the distinctive parliament building known as the "Beehive".