Aftershocks are continuing to shake Wellington and the top of the South Island after a 6.5 magnitude quake just after 5pm today.
The big tremor has been followed by a number of smaller quakes ranging from light to strong.
What you need to know:
• Four people have received minor injuries.
• People are advised to stay at home and not travel unnecessarily - while checks are done on roads and buildings and other structures.
• Police have closed sections of Featherston Street, Wakefield Street and Bolton Street in the CBD and may close other streets if hazardous.
• Wellington Airport has been fully re-opened but there is likely to be a backlog of delayed flights
• The region's rail network is closed while checks are done on bridges, tunnels and other structures. There are no bus replacements.
• There have been some reports of damage to CentrePort.
• The Fire Service is busy dealing with broken water pipes and, for example, stuck lifts.
• At the moment there has been one report of a small electricity outage in Wainuiomata - but other services appear to be unscathed.
Did you feel the quake? Send your story, photos or video here.
The big quake today, which was centred 30km east of Seddon, the site of all the seismic activity in the last couple of days, was 11km deep.
This afternoon's large shake, which struck just after 5pm, followed dozens of aftershocks after this morning's magnitude 5.8 in Cook Strait between Wellington and Seddon at 7.17am.
A magnitude 5.7 quake shook the region on Friday.
REPORTS OF DAMAGE
Reports of damage around the Wellington region varied from falling masonry and broken windows to water leaks and stock falling from shop shelves.
Glass from broken windows fell onto Lambton Quay and street workers were keeping people away from those buildings in case another quake struck and loosened the glass that was left.
A roof was damaged at Harvey Norman in Paraparaumu and a wall collapsed in Wainuiomata, causing a gas leak.
Wellington Region Civil Defence Controller Bruce Pepperell said there had been reports of structural damage to a number of buildings around Wellington and emergency services and local authority staff were continuing to check buildings and infrastructure.
"At the moment we have had only one report of an injury around the region."
The Wellington Region emergency management office was activated this evening, as were those in Porirua and Hutt Valley, although it appeared damage was minor in those areas.
Mr Pepperell said while the earthquake was undoubtedly frightening, it did not appear to have done widespread damage.
"While some buildings are damaged and have been evacuated, the city and region has by no means ground to a halt."
He advised residents to check up on their neighbours - and go and stay with friends, family or neighbours if they were frightened.
The Pacific Tsunami Warning Centre (PTWC) said Civil Defence had made an initial assessment that the earthquake was unlikely to have caused a tsunami that would pose a threat to New Zealand.
There were reports of four injuries around Wellington but all were minor.
CBD workers were advised not to go into work until midday to allow building checks to take place.
Mr Pepperall said said there were a number of buildings in the CBD with damaged and potentially dangerous facades and there was probably damage to other buildings and infrastructure that had not been discovered this evening.
"If at all possible, people should stay home _ and contact their managers and employers first-thing in the morning to get advice.
"We need to give building owners, building managers, tenants and engineers time to do checks on buildings to look for damage and make sure they are safe.''
"If at all possible, stay home - and keep tuned to the media to stay up to date with the situation.''
"It is likely there will still be some parts of the CBD cordoned-off in the morning _ so in some cases, people might not be able to get access to their workplaces.'"
Police had closed sections of Featherston Street, Wakefield Street and Bolton Street in the CBD, along with Willis and Webb streets and may close other streets if hazardous.
New Zealand Post House on Wellington's Waterloo Quay would be closed tomorrow while structural checks were made.
Staff and customers were advised that they would not have access, including to the box lobby.
Some parts of the building had damage to internal fittings and fixtures, a spokeswoman said.
ANZ branches in Wellington's central business district would be closed to customers for most of tomorrow while ANZ had them checked by engineers.
"Subject to the availability of engineers, we're hopeful that these branches will open later on Monday.
The safety of our customers and staff is important to us. We apologise to customers for this inconvenience.
We will have updates on our website when they come to hand,'' a spokesman said.
KiwiRail said all Wellington commuter lines would remain closed tomorrow morning with no peak train services or bus replacements operating.
Tranz Metro passengers were advised to make alternative travel arrangements for the morning.
Tranz Metro train services were suspended immediately following the magnitude 6.5 earthquake which struck at 5.09pm.
Cursory track inspections had been undertaken across all four lines (Wairarapa, Kapiti, Hutt Valley and Johnsonville) to check for damage to the rail.
However, due to the number of aftershocks that had taken place and the constraints involved in undertaking inspections without daylight, KiwiRail would be carrying out another inspection tomorrow morning to ensure that there was no damage to the tracks, bridges and tunnels that may not have been identified in the dark.
Wellington Airport has now been fully re-opened but there is likely to be a backlog of delayed flights
Wellington Airport spokesperson Briarley Kirk said the size of the earthquake meant the airport needed a full runway and navigational aid inspection as part of standard operating procedure.
The runway was inspected and cleared for operation by 5.55pm with a number of aircraft being able to operate shortly afterwards. All navigational aids were inspected and cleared by 7.30pm. Some flights were cancelled or delayed during this period.
She advised passengers to check with airlines or on the Wellington Airport website for the status of their flights.
Jeremy Ward of the East By West Ferries, which operates daily services between Queen's Wharf in central Wellington and Day's Bay at Eastbourne, said when the earthquake happened one of the boats was on the water.
The skipper found out when passengers started reading social media and receiving texts.
Mr Ward was in the ferry headquarters at Queen's Wharf, which suffered some damage.
"We had a bit of superficial damage, the paint all cracked on the roof, and there's a bit of paint on the floor. The drinks fridge also tipped over on the floor."
A spokeswoman for the Interislander ferry - travelling between Wellington and Picton - said there was no disruption to services.
A spokesperson for the Bluebridge Cook Strait Ferry, said the company's services had not been affected.
Marianne Aitken, spokeswoman for the Marlborough District Council, said there had been no power in Seddon since the earthquake.
An evacuation centre was being set up at the Awatere Rugby Football Club from 8.30pm for residents to take shelter.
She had not yet heard of any injuries.
Wellington City Council spokesman Richard MacLean said he had received reports of power going off for a time in the suburb of Karori.
"No one's heard of any injuries as of yet. But there's is quite a lot of evacuations going on in various buildings around town . Some buildings, we are aware that there is significant damage to them. But at the moment, the city is pretty much still running.
"Electricity's on, people are still in restaurants and bars... it is not a chaotic situation by any means."
Emergency services were gathering at the regional emergency management office in Thorndon.
"We're getting reports of damage coming in," Mr MacLean said.
"We're obviously working with the fire service and the police. They're the ones who are going to be giving us a lot of the information.
"We're getting lots of reports of stuff coming off shelves, structural damage of buildings."
A Fire Service spokeswoman said nearly 50 calls were received to the central communications centre in the half hour after the earthquake.
All 12 Wellington appliances were sent out on jobs, with a backlog starting to pile up by 5.45pm.
She said there were multiple alarm activations in central Wellington with widespread reports of structural damage.
St John Ambulance did not attend any jobs related to the earthquake, a spokesman said.
There had been no admissions to Wellington Hospital as a result, a Capital and Coast District Health Board spokeswoman said.
WELLINGTON'S CENTRAL LIBRARY
There were cordons around Wellington's central library after a water leak.
Wellington city council director of property and housing Greg Orchard said connection between the library and Wellington City Council building had been cordoned off for safety reasons.
Mr Orchard said cracks have appeared either side of the two-level connecting structure. The structure is unsupported from below, sitting in between buildings.
Water is leaking from the structure but the source of the leak had yet to be determined.
An engineer had been called in but he would inevitably be very busy, Mr Orchard said.
Office worker Alice Midgley was on the 13th floor of the Fujitsu building when the quake struck.
"I was under the desk, that was the first time I was frightened," she said.
April Ferrino from Austin, Texas, was in a fifth-floor apartment on Lambton Quay.
"I'm from Austin, Texas so we're used to other natural disasters - tornadoes, hurricanes. Earthquakes are extremely terrifying because you can't predict them," she said.
"Things started falling off the shelves. It was terrifying."
"I felt the first tremor this morning which was a slow rumbling.....but this one was a jolt. It was extremely terrifying."
She left her building when it happened and said she would go back up to pack a bag in case there was another big one and she had to leave.
Steve Mabin was on the 11th floor of the Prudential Building on Lambton Quay.
"I was sleeping so all I heard was smashing happening so I ran to the door. I just saw the door swaying so I ran for my life [downstairs]. That's a lot of stairs to run down."
Residents in homes perched on the hills around the capital were also shaken.
"That's the worst quake I've ever felt," said resident Sarah Bennett.
Ms Bennett and her husband Lee Slater, whose Highbury house is built on the side of a hill, said it was swaying on its piles.
"You usually only get a bit of a jiggle but this was a hula dance," Ms Bennett said.
Mr Slater said he was in shock.
The quake was also widely felt elsewhere.
Deborah Parsons, who lives in Blenheim, felt the quake and said it was scary.
The contents of her china cabinet had smashed on the floor.
"I'm getting the emergency kit ready and I'm packing up what's left of the china," she said.
New Plymouth resident, Michael Riley, told the Herald that "everything was moving" in his Taranaki home.
"There was 20 seconds of shaking, everything in the room was moving," he said. "I went outside and still all the power lines were shaking and it was just chaos, it was quite scary. It was the biggest earthquake I have ever felt."
Andrew Feldon said: "Felt it here in Hastings! Kept going for ages - was watching the trees outside the window swaying."
Social media users were quick to report their experiences of the quake.
"Felt in Te Awamutu," said one Twitter user. "Trees were rocking and hanging plants swaying - felt seasick."
Another said: "They are coming in on waves"
Hope you're all OK after the shake-up, Wellington! There was no damage to the Zoo that we can see at this point & animals are fine #eqnz— Wellington Zoo (@WellingtonZoo) July 21, 2013
Unfortunately This does mean an evening of aftershocks but as they lesson you shouldn't feel too much shaking. #eqnz— GeoNet (@geonet) July 21, 2013