A powerful earthquake and series of aftershocks shook New Zealand early on Monday, killing at least two people and prompting a tsunami warning that sent thousands fleeing to higher ground.

Emergency response teams were dispatched by helicopter to the region at the epicentre of the 7.8 magnitude quake, some 91 km (57 miles) north-northeast of Christchurch in the South Island, amid reports of injuries and collapsed buildings.

Prime Minister John Key told a dawn news conference in the capital of Wellington, where the quake was also felt strongly, that two people had been killed. Police said one of the victims was found in a house in the coastal tourist town of Kaikoura.

"It was the most significant shock I can remember in Wellington," Key told reporters, although he stopped short of calling a national emergency. "There will be quite major costs around roads and infrastructure."

Emergency officials were meeting later on Monday morning, added Key.

Power was out and phone lines down in many areas of the country, while roads were blocked by landslips. But a tsunami warning that led to mass evacuations was downgraded after large swells hit Wellington, in the North Island, and Christchurch, the South Island's largest city.

The first tremor, just 23 km (14 miles) deep, struck the Pacific island nation just after midnight, jolting many from their sleep and raising memories of the 6.3 magnitude Christchurch quake in 2011, which killed 185 people. New Zealand's Geonet measured Monday's quake at magnitude 7.5, while the U.S. Geological Survey put it at 7.8.

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.


RESCUE PERSONNEL

St. John Ambulance said it was sending helicopters carrying medical and rescue personnel to Kaikoura, where at least one of the casualties was located. The South Island town, a popular destination for whale watching, was completely cut off and officials said there were reports of a collapsed building.

In Wellington, where frequent aftershocks continued to be felt hours after the first quake, residents heading for higher ground caused gridlock on the roads to Mount Victoria, a hill with a lookout over the low-lying coastal city.

"I'm just sort of parked by the side of the road and I think people are trying to go to sleep the same as I am," Wellington resident Howard Warner told Reuters after evacuating his seaside house.

Richard Maclean, a spokesman for Wellington City Council, said there was structural damage to several buildings.

Residents were advised to stay away from the central business district on Monday and the train network was closed for checks. Wellington International airport, however, was expected to open as usual on Monday.

In Christchurch, where tsunami sirens continued intermittently, three evacuation centres were accepting residents. Police set up roadblocks to prevent people from returning to lower-lying coastal areas.

Pictures shared on social media showed buckled roads, smashed glass and goods toppled from shelves in shops in Wellington and the upper South Island.

"The whole house rolled like a serpent and some things smashed, the power went out," a woman, who gave her name as Elizabeth, told Radio New Zealand from her home in Takaka, near the top of the South Island.

There was initial confusion when emergency services first said there was no tsunami threat.

Christchurch Civil Defence Controller John Mackie said that while the earthquake was centred inland, the fault line extended offshore for a considerable distance. That meant that seismic activity could cause movement out at sea, leading to a tsunami.