The epicentre of the strongest quake was 91km northeast of Christchurch
A tsunami has hit New Zealand's South Island after a series of powerful earthquakes struck across the centre of the country.
New Zealand police have confirmed there has been one death following the quake. The tsunami warning has since been downgraded.
The country’s Civil Defence Minister Gerry Brownlee confirmed to The Guardian the first casualties from the earthquake in the town of Kaikoura, on the east coast of New Zealand’s South Island.
New Zealand's civil defence warned people to move to higher ground as the tsunami threat may last for several hours.
The initial earthquake hit northeast of Christchurch around midnight local time (11am Irish time), the US Geological Survey reports.
The USGS says that the centre of the quake was on land, 91km from the city which was badly damaged by one that measured 6.3 in 2011.
It was initially measured 7.4 magnitude, but later revised upwards to 7.8 by the USGS. New Zealand's Geonet puts the magnitude at 7.5.
New Zealand media reports that local ambulance teams are responding to reports of injuries in the earthquake area. No further details are yet available.
The initial tremor was followed by several aftershocks, further north and also both inland - including a 6.2 magnitude about 28km northwest of Kaikoura and a 5.7 south-southeast of Bleinheim.
New Zealand's Geonet, which describes itself as the official source of geological hazard information for the country, listed dozens of quakes between 6.6 and 3.8 after midnight local time.
There have been widespread reports on social media of damage to buildings and other structures.
Tsunami alarms have been sounding in coastal areas.
On their website, the NZ Civil Defence warned: "The Ministry of Civil Defence & Emergency Management (MCDEM) has issued a tsunami warning (marine and land threat) from East Cape to Southland, including Wellington (which also includes the Cook Straight area), Marlborough Sounds/Tasman Bay and the Chatham Islands, and a marine threat for all other New Zealand coastal areas.
"Current assessments indicate that waves of up to 3-5 metres along the coastlines closest to the earthquake epicentre (estimated from Blenheim to Banks Peninsula, and also including the Chatham Islands) can be expected in some areas. The remainder of the east coast can expect waves of up to 1-3 metres," it adds.
The relatively shallow first quake occurred at a depth of 10km. Shallow tremors are said to be felt more strongly on the surface.
Strong jolts were reported over 200km away in the capital, Wellington, where at least one tweeter posted pictures of damage to their home.
Witnesses in Christchurch have said that the tremors lasted for a longer time that previous quakes but were less intense.
It is thought the epicentre is not densely populated but is popular with tourists.
The February 2011 tremor was just a few miles outside of New Zealand's third biggest town and caused widespread damage and left 185 people dead.