There have been two fatalities after a series of huge earthquakes overnight, Prime Minister John Key says.
Authorities were scrambling this morning to assess damage and respond to reports of injuries after the first 7.5 magnitude earthquake centred near Hanmer Springs.
Watch: Prime Minister John Key media conference
The Ministry of Civil Defence and Emergency Management confirmed there had been two deaths from the earthquake.
One person died in a house that collapsed in Kaikoura, and a second person died at a house in Mt Lyford, north of Christchurch.
Mr Key said a Defence Force helicopter would be flying to Kaikoura and another one would be available in Wellington.
An Urban Search and Rescue team from Christchurch was also on its way to Kaikoura by helicopter, along with two further assessment teams deployed to Waiau and Blenheim.
As a result of the quake, Mr Key cancelled his proposed trade trip to Argentina, and will decide later this week whether to attend APEC in Peru next weekend.
Nelson Marlborough District Health Board said five people had been admitted to Wairau Hospital by early this morning with various injuries, some described as moderate.
Follow our full coverage:
- Earthquake: What you need to know
- LIVE: Earthquake updates as they happen
- Scores of aftershocks rattle country
- Kaikoura cut off by road
- Earthquake: In pictures
The first quake struck at 12.02am, near Hanmer Springs, at a depth of 16km. It was followed by a series of large earthquakes across New Zealand up to 6.2 in magnitude, which prompted tsunami alerts, causing residents of coastal areas to flee inland.
GNS science said there had been at least 250 aftershocks since the massive shake just after midnight, with more than 30 of them above magnitude 5, and three of magnitude 6 and above.
Most were in the upper South Island, in Seddon and Kaikoura, but a 4.9 magnitude earthquake hit Paraparaumu and a 4.9 hit Wellington in the early hours.
The Civil Defence ministry said roads were closed on State Highway 1 between Blenheim and Picton, State Highway 6 between between Blenheim and Nelson, and State Highway 7 between Springs Junction and Waipara.
It warned people to check for any dangers on damaged properties including to gas and water supplies.
The earthquake sequence is the biggest since the 5.7 Valentine's Day earthquake, which caused cliffs in the Christchurch suburb of Sumner to fall and liquefaction in some east Christchurch areas.
Tsunami activity was detected in Kaikoura and Wellington, with initial tsunami warnings prompting sirens from Northland to Christchurch, and waves detected in Wellington and the South Island.
A tsunami at least 2m high was recorded near Kaikoura, Civil Defence tweeted.
The Ministry of Civil Defence and Emergency Management has now cancelled the tsunami warning that was in place from Wellington to Banks Peninsula.
But it said coasts may still experience unusual, strong currents and sea level fluctuations lasting for several more hours.
KiwiRail said all trains on the North Island Main Trunk line, south of Palmerston North, and in the South Island had been cancelled until further notice.
Ferry sailings were also cancelled until a full assessment could be made of the terminals in Wellington and Picton.
The Ministry of Education advised schools and early childhood centres from North Canterbury through to Wellington would remain closed today.
It also said all NCEA Scholarship exams would be suspended.
Houses were reported to be damaged in Kaikoura, with chimneys down in Culverden.
Police earlier said they were trying to get access to the Mt Lyford property where one person was thought to have died.
Three evacuation centres were open in Christchurch. At 5am there were 230 people at Linwood College, 50 at Mairehau School and 25 at Akaroa School Hall.
Hotel guests in central Christchurch were evacuated. More than 100 people stood on the footpath opposite the Christchurch Casino, while at least 100 hotel guests were outside the Novotel, opposite the Christ Church Cathedral.
Alan Vincent was on the water near Lyttelton near Christchurch when the quake struck. He said it felt like "giants shaking the living daylights out of the boat".
Cheviot firefighter Chris Hill said the roads were "a bit cut up".
A temporary boil water notice has been issued in Hurunui. A welfare centre was set up at the Amberley Pavilion.
Christchurch police have set up roadblocks to stop people going to coastal areas while a tsunami warning was in place.
Robin, from Christchurch, said the earthquake was a "wake-up call".
New Brighton resident Dean Kozanic evacuated his home as a precaution because of the tsunami warning. He said many residents in the suburb had been doing the same.
Power was cut in North Canterbury, from Kaikoura to Cheviot. The Hurunui District Council said Rotherham and Waiau were isolated by slips and damage to the bridge.
Northern South Island - train trapped
Marlborough Civil Defence controller Richard McNamara said a train had been trapped north of Kaikoura this morning.
"Our helicopter will drop in and check on the driver, we've been talking to train control South Island," Mr McNamara said early this morning.
"There'll be other people on the road that will be stuck and obviously the heavy traffic between Picton [at] the interisland ferry and Christchurch will also be held up."
He said bridges north of Blenheim had been damaged, but there was a diversion in place.
Fiona Redfern, of Muzzle Station on the Clarence River, warned neighbours on Facebook that the Dart Stream tributary had become dammed due to a slip and a flash flood could come at any time.
Several people spent hours stuck on the Interislander Ferry, which was unable to dock in Picton due to a power cut. Other ferry movements were suspended for the day.
Marlborough Civil Defence said buildings in Picton's main street were reported to be badly damaged, and the earthquake had caused liquefaction on the Picton foreshore.
Twenty people were receiving welfare help in the Ward Town Hall in Marlborough.
The area from Seddon to Ward had been checked by helicopters and some damage to electrical transmission poles had been found which could take days to repair.
There was a diversion in place on SH1 between Picton and Blenheim, which was open for some vehicles but heavier vehicles could not get through and were reported to be banked up for several kilometres along the highway.
Picton residents were asked to conserve water as one of the town's reservoirs was leaking.
An evacuation centre was being opened at the Waikawa Marae.
The Nelson Marlborough District Health Board said by early this morning five people had been admitted to Wairau Hospital with various injuries, some described as moderate.
Spark said its phone networks in Clarence, Kaikoura, Weld Cone, Waiau and Eltham in the South Island have been affected. People should text rather than call if possible, it said.
Power went out in many areas including Picton, Blenheim and Havelock.
Lansdowne Park in Blenheim was also closed due to liquefaction.
Wellington City Council warned people not to go to work in the central city and Lower Hutt until the evening.
Several multi-story buildings received damage - both internally and structurally - and glass fell into streets.
Power was out in some areas, and Civil Defence controller Bruce Pepperell said early indications were that a number of major buildings showed signs of structural stress and the strong quake would likely have caused a mess inside.
Mr Pepperell said the capital's suburban rail network would not be operating until further notice, while rail tracks, bridges and tunnels were checked. Buses were unlikely to replace the trains.
He said 25 buildings in the CBD had so far been identified as having suffered potential structural damage.
"Certainly there's indications of some structural stress down in some of the buildings down in the CentrePort.
"There's a little bit of liquefaction down there, well, a lot of the CBD's on reclaimed land."
The damage had been referred to building owners for detailed examination by engineers, Mr Pepperell said.
Some city streets were showered with broken glass during the quake and it was feared strong winds tonight could dislodge more glass.
Land movement of up to 1m has been reported on reclaimed land on the waterfront with large cracks opening up in concreted areas.
Wellington Harbour Master Mike Pryce said he and port operator CentrePort were still assessing the earthquake damage to the waterfront including liquefaction.
CentrePort said it would issue an update on the situation shortly.
Victoria and Massey university campuses in the capital were closed this morning, with Massey saying exams would be postponed until further notice.
People who left their homes in the city centre were told they could find temporary refuge at the BizDojo Centre on 115 Tory Street, or Houghton Valley School further out of the city.
Residents in coastal areas were urged to stay inland or on high ground. Affected spots include seaside areas of Wellington's south coast, Seatoun and Eastbourne.
The state of emergency in Dunedin has been lifted after Civil Defence evacuated about 50 households in seaside communities.
Civil Defence in Otago said the land-based threat to the eastern coasts had passed and people were being allowed to go home.
The controller for Clutha district, Charles Hakkaart, said the main evacuations were in low-lying houses in Pounawea, Taieri Mouth and Toko Mouth.
About 40 people were evacuated from their homes on the Chatham Islands, mainly in the settlement of Kaingaroa, and more made their own way to higher ground.
Chatham Islands mayor Alfred Priest said the island was the path for tsunamis from both New Zealand and South America, and locals were good at responding to the threats.