A series of earthquakes have terrified many already-frazzled Christchurch residents this afternoon, starting with a 5.8 magnitude tremor at 1.58pm.
It was followed by a 6.0 magnitude aftershock at 3.18pm, centred 10km east of Christchurch in South New Brighton.
There was also a 5.3 magnitude quake at 2.06pm and a 5.0 at 4.50pm.
Initial reports said one person had received minor injuries, although St John had responded to 19 earthquake-related emergency calls in and around Christchurch.
They ranged from complaints of chest pain and seizures, to a panic attack.
Five people had collapsed, and one suffered a knock to the head.
St John said all injuries were minor.
Police say there is no tsunami threat, and they have no reports of serious or widespread damage or injury.
Civil Defence staff have met on the ground in Christchurch to get a good picture of the extent of impact, but national civil defence controller David Coetzee said it was too early to comment on the way forward.
He said while there were no reports of serious damage, buildings that were already weakened in previous earthquakes may have sustained further damage.
"There will be further aftershocks. This event pretty much takes the pattern of the June event," he said, referring to a strong aftershocks in Christchurch that month.
"Be aware of further aftershocks. Drop, cover and hold remains the old mantra. Look after each other, look after the neighbours, stay safe and call 111 or the council if you need help."
Rockfalls and damage
People were urged to stay away from hill suburbs because of risk of rockfalls, and Coastguard had rescued four people who were trapped by rockfall in Boulder Bay.
Police said there had also been a significant rockfall at Redcliffs, a stopbank on New Brighton Road had collapsed, a partly demolished building on the corner of Barbadoes and Kilmore Streets collapsed, and a vacant house at 81 Aynsley Terrace collapsed.
Some residents at Windsor Hospital rest home in New Brighton were trapped on the second floor of the building, but are being evacuated by the Fire Service.
There has been significant flooding and liquefaction in the eastern suburbs including Avonside and Bexley.
The India Blue restaurant in Sumner is damaged and has been cordoned off and a building on the corner of Charles Street and William Street in Kaiapoi is at risk of collapse.
Road damage and liquefaction
Major holes in the road were reported to have opened up on Broadhaven Avenue, and liquefaction was reported in Avonside.
Drivers were urged to slow down and be patient as congestion built up in some areas.
Police said Christchurch residents should stay off roads if at all possible, and text rather than phone.
Christchurch Hospital, the Princess Margaret Hospital and Hillmorton are all fully operational. Burwood is operating on emergency power.
There are no reports of damage and no injuries to staff or patients who were at the board's facilities at the time of the quake.
The DHB said people can access 24-hour non-emergency care through their general practice, and are asked to help keep the Emergency Department clear for genuine emergencies. The 111 service is operating normally for emergencies.
The Christchurch mayor said his heart breaks for Cantabrians shocked by a series of earthquakes this afternoon.
The first, a magnitude 5.8 tremor, struck at 1.50pm. Its focal point was at a depth of eight kilometres, 20km off the coast of Lyttelton.
Christchurch Airport remains evacuated, with literally hundreds of people standing outside with their luggage.
Newstalk ZB reporter Scarlett Cvitanovich says nobody knows when they'll be able to go back inside.
"There's quite a bit of aggression out there. People are asking how they're going to get on flights in time for Christmas and are taking it out on staff at the moment," she says.
Flights bound for Christchurch are also being diverted to other cities.
Key: 'My heart goes out to the people of Christchurch'
Prime Minister John Key said Canterbury Earthquake Recovery Minister, Gerry Brownlee, will be heading to Christchurch this afternoon to access the situation.
"My heart goes out to the people of Christchurch and Canterbury at this time," Mr Key said.
"However, residents can be confident that the authorities are onto the situation and government resources stand ready to assist wherever they are needed.
"The Government's resolve to work with the people of Christchurch and Canterbury to rebuild remains unchanged following today's two aftershocks."
Mayor Bob Parker has now booked to fly back to Christchurch from Taupo, where he was holidaying.
He told TVNZ he was getting a lot of reports in from the east of the city of minor damage.
"On the east there's more reports coming in of liquefaction, in Avondale ... and there's small collapses around riverbanks."
He said while initial reports tell of minor damage to the city, today's shake would have taken a large emotional toll on people.
"Many of them just sitting around and in tears. You just can't understand the ongoing stress."
"We were just getting over that feeling, I think, and it sets people back."
Chlorine had finally been removed from the waterways two weeks ago, and today's quake could have set infrastructure progress back, he said.
'Very violent... constant rolling'
Jacqui Taylor said she was working in the Lyttelton library when the first magnitude 5.8 quake hit.
"It was very violent, with constant rolling for a very long time, we left the building and people gathered in the main street. there were more aftershocks while we were in the street.
"I luckily made it through the tunnel and back home in Addington, feeling one more shake while i was in my car at the lights."
Electricity distributor Orion said, following the 3.18pm magnitude 6.0 aftershock, power was again tripped to the Eastern Suburbs of Christchurch and to the rural central Canterbury area around Springston.
Around 26,000 customers remain without power in eastern Christchurch. Power is out from the New Brighton area through to Shirley.
Orion said its crews were out assessing damage. It said it appeared power is out due to tripping caused by shaking rather than damage to equipment.
Neanwhile New Zealand Post said there will be no mail deliveries in Christchurch. The Christchurch Mail Centre has been evacuated as a precaution and will not re-open this evening.
The next scheduled mail deliveries following the statutory holidays will be December 28.
Emergency Operations Centre
The Christchurch City Council has established an Emergency Operations Centre in the city. There has been a rockfall behind containers at Scarborough, and liquefaction at Parklands, the council said.
Police are doing aerial reconnaissance to check for damage across the city.
All Council buildings will be closed until they can be assessed as safe to occupy. People should assume all Council buildings are closed until they can be assessed and declared safe.
Cantabrians are being warned that more aftershocks could be on the way before Christmas.
GNS: More aftershocks to come
GNS seismologist Ken Gledhill told TVNZ "this is part of a series of earthquakes that have been set off by what happened in September 2010.
"In the coming days the most likely scenario is that there will be a series of aftershocks in a similar location.
"This is just a reminder that this area is more active than what it was before September 2010."
He said GNS were doing the best they could with information and the situation in Christchurch.