An earthquake with a magnitude of 7.8 struck central New Zealand just after midnight, the US Geological Survey said, causing widespread damage and generating a tsunami.
New Zealand's Ministry of Civil Defence and Emergency Management warned of the risk of destructive waves along the country's entire east coast and urged residents in low-lying areas to evacuate and seek higher ground.
Waves of up to two metres (6 feet) could be possible for up to two hours, it said.Anna Kaiser, a seismologist at GNS Science said a tidal signal or surge of up to one metre (3 feet) had been recorded in North Canterbury region of the South Island.
"That's reasonably significant so people should take this seriously," she told Radio New Zealand.
New Zealand's Geonet revised up its estimated magnitude of the quake to 7.5, from 6.6 earlier.
USGS said the quake was centred 91 km (57 miles) north-northeast of Christchurch, the biggest city on New Zealand's South Island.
A 6.3 quake there in February 2011 killed 185 people and caused widespread damage.
The tremor was felt throughout most of New Zealand. Some injuries had been reported but there no immediate reports of deaths.
"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.
Chris Hill, a fire officer in Cheviot, a coastal town near the quake's epicentre, said officials had gone door to door evacuating residents.
"Everyone seems OK here," he told RNZ. "There's a lot of debris in houses, but at this stage it doesn't look like anything too bad has happened."
Residents in Wellington said glass had fallen from buildings into the streets and hotels and apartment buildings had been evacuated.Richard Maclean, a spokesman for the Wellington City Council, said residents had been evacuating the city's southern coastal suburbs due to the tsunami threat and there had been reports of structural damage to several buildings.
"We've got reports of broken water pipes and lots and lots of things off desks and shelves," he told RNZ.
Several roads around the South Island had been closed and damage to power and phone networks was reported.
A series of aftershocks were recorded around the country, some as strong as 6.1 magnitude.
Pictures shared on social media showed buckled roads, smashed glass and goods toppled from shelves in shops in Wellington and the upper South Island.