A powerful 7.8 magnitude earthquake pummeled central New Zealand early on Monday, killing at least two people, damaging roads and buildings and setting off hundreds of strong aftershocks.

Emergency response teams flew by helicopter to the region at the epicenter 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. There were concerns that loose glass and masonry could be dislodged by severe weather hitting the capital, with 140 km per hour (85 mph) winds forecast.

Hundreds of aftershocks, the strongest a 6.2 quake at about 1.45 p.m. 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, centered 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, appeared to have borne the brunt of the quake