18:32: Government research institute GNS Science has warned of more aftershocks for Canterbury people in the coming days and weeks.
The two major jolts to hit Christchurch this afternoon were within the range of forecasted aftershocks as modelled by scientists, GNS Science said.
The magnitude 5.5 and 6.0 aftershocks that hit at 1pm and 2.20pm were centred on the coast about 10km south of Christchurch and a short distance south of the fault that ruptured in the 6.3 quake on February 22, killing 181 people.
"These aftershocks were within the existing Canterbury aftershock zone and were within probabilities," said Kelvin Berryman, manager of the GNS Natural Hazards Research Platform.
"This size of events is likely to produce its own aftershock sequence, therefore rejuvenating aftershock activity at least in the short term.
"We would expect a number of aftershocks in the magnitude 4.0 to 5.0 range on the coming days and weeks," Dr Berryman said.
17:59: Countdown Northlands, Bush Inn, Kaiapoi, Avonhead, Moorhouse, New Brighton, Northwood and Church Corner are currently closed and are expected to reopen during the course of tomorrow, following engineer and safety checks.
The Colombo St store remains closed until further notice. Countdown Eastgate was evacuated and re-entry is subject to engineer checks.
The Hornby and Rangiora stores remain open and are trading.
Countdown Ferrymead and The Palms remain closed from the February 22 earthquake.
17:42: Westfield New Zealand confirms the Westfield Riccarton shopping centre has remained closed for the rest of the trading day due to the aftershocks that took place today. We evacuated the shopping centre after the first significant aftershock occurred. Our decision to keep the centre closed for the balance of the day is a prudent one given the severity of the continued aftershocks. We anticipate reopening the shopping centre for trade tomorrow morning said Mr Justin Lynch, Director, Westfield New Zealand.
17:22: Lyttelton's historic Timeball Station's tower has collapsed following the magnitude 6.0 earthquake that struck Christchurch at 2.20pm today. While some of the building remains standing, the site was too dangerous to enter to make any assessment of the extent of the latest damage, The New Zealand Historic Places Trust (NZHPT) said. Work had begun on dismantling the property two weeks ago, with ballasted containers and straw bales placed around the site. The immediate area had already been fenced off following the February 22 earthquake that severely damaged the 134-year-old landmark.
16:26: School buses are operating as normal for Avonside Girls, Linwood College, Marian College, St Paul's Dallington, Shirley Boys' High and Catholic Cathedral College, the Ministry of Education has reported.
15:52: Eleven Countdown supermarkets across Christchurch have been closed following this afternoon's earthquake. Countdown Colombo St is closed until further notice, while Countdown's Eastgate store has been evacuated with re-entry subject to engineer checks. Countdown's Kaiapoi, Avonhead, Moorhouse, New Brighton, Northwood and Church Corner stores are expected to reopen tomorrow. Other stores in the group, including Bush Inn, Northlands and Rangiora are expected to open later today.
15:32: The Ministry of Civil Defence and Emergency Management has activated the National Crisis Management Centre following today's major aftershocks. National controller, David Coetzee, said the response to the aftershocks is being managed from the Christchurch City Council's Emergency Operations Centre. Civil Defence said there are unconfirmed reports 10 people have been reported injured and there have been no fatalities reported to date,
The police headquarters and the offices of the Christchurch Earthquake Recovery Authority have been evacuated.
15:16: Christchurch East MP Aaron Gilmore said he drove from his office to his home in his electorate after the 5.5 magnitude quake. He said the areas which were heavily hit by liquefaction after the February 22 earthquake - Parklands, New Brighton, Bexley, Aranui - once again have flooded streets. "The damage - there is mud, liquefaction water and potholes all along the streets that have already been repaired in the last few months. "The poor guys who have been doing all that work - well they're out there already."
"It is pretty serious. He saidd the magnitude 6 quake felt much stronger.
"There will certainly be some injuries from these quakes - hopefully there are no major serious injuries."
15.04:Fire Service spokesman Ian Lynn said a bridge has reportedly come down following the magnitude 6 quake - believed to possibly be on Gayhurst Road, Avonside. He said there have been further callouts, mostly for broken water mains. "This place is going mad."
15.04: Orion said power has been cut to around 54,000 customers across Christchurch following the magnitude 6 earthquake.
14.37: A caller told Newstalk ZB's Danny Watson that water is pouring out of buildings on Stanmore Road. "It's a funny colour. It looks like sewage."
14.32: Christchurch has been rocked by another aftershock, which some have said felt worse than 5.5 magnitude one that struck at 1pm. The latest one struck around 2.20pm. It was felt widely around the South Island.
14.30: Telecom said its mobile and fixed network are operating well, but there is some congestion. Immediately after the aftershock three cell sites when down as they lost power, but are now all operable. Cellphone users are asked to use texts rather than making calls.
14.09: GNS Science seismologist Jennifer Coppola said the earthquake would have felt stronger than last Monday's 5.5 magnitude aftershock, as it had a stronger peak ground acceleration. Ms Coppola said GNS Science had forecast between one and 10 aftershocks between magnitude 4 and 4.9, and between zero and two larger than magnitude 5 for Canterbury between May 19 and June 18. "So we've hit the magnitude 5 and above number - hopefully there are no more."
13.58: Orion is reporting power has been cut to 10,000 customers in east Christchurch, from Heathcote to Brooklands including Bromley, Belfast and Marshland. Power has been restored to parts of Dallington and Avonside, Orion said, and is expected to be restored to the city by nightfall.
13.58: Christchurch police spokesman Stephen Hill said there were fears people were trapped inside a collapsed unit on the corner Stanmore and Worcester street. He said emergency services had been called to clear the rubble from the the building site.
13.57: Fire Service spokesman Ian Lynn said two people have been rescued from the St John Church on Hereford and Madras streets. Mr Lynn said crews have been called out to around 38 incidents, including a house fire, bust water mains, and liquefaction.
13.49: Christchurch Cathedral dean Peter Beck said the "very big" aftershock had shaken his temporary offices above Christ's College. Students had been evacuated and were gathering on the school fields, he said. He had received reports about damage to homes and more damage to the city centre, but no word of injuries. "We just hope and pray that no-one's been hurt. "It's a hard time but at the same time we are an extremely resilient people. We will live through it. We will continue to recover and rebuild our city." The aftershock would have inflicted further damage on the severely earthquake hit Christchurch Cathedral, Mr Beck said.
13.47: Fire Service spokesman Ian Lynn said two people have been rescued from the St John Church on Hereford and Madras streets. Mr Lynn said crews have been called out to around 38 incidents, including a house fire, bust water mains, and liquefaction.
13.36: Christchurch Mayor Bob Parker said the earthquake felt at least as powerful as the February 22 quake, but much shorter in duration.
He had received early reports of buildings buckling in the city centre and was still waiting to hear whether there were injuries to staff working in the area.
The council building had been evacuated, he said.
"I must admit I was very concerned. Our building was rattling and shaking. Trees in the streets were waving at crazy angles."
The quake would have brought back traumatic memories for many residents, Mr Parker said.
"It doesn't even take a 5.5 magnitude to bring back traumatic memories. Any quake brings it all back. All your can do is wait. That's all. You can't move."
13.35: A Facebook user writes: "Not enjoying being up the hill above sumner right now... Saw half a bloody cliff fall from that shake."
13.34: Joy Reid, of TVNZ, tweeted about being in Riccarton mall.
"Stuff fell off shelves.People crying. Everyone leaving," she tweeted.
13.33: The earthquake has impacted both the cellphone and landline network. Telecom says it is assessing any potential network impact.
13.30: Andrew Leckie from the suburb of Shirley says the power is out, with more damage around the house. He says "it felt as big as the Feb22nd quake here!".
13.27: There are reports of fresh damage in the Christchurch's CBD. One caller to Newstalk ZB described the quake as "huge".
13.27: Initial reports are that phone lines are down and rocks are falling off Sumner's hills.
13.20: Buildings have been rocked throughout Christchurch in what is being described as the biggest aftershock to hit the devastating February 22 quake.
Residents have taken to Twitter to post about the quake, with many describing it as powerful and estimating it at a near 6 magnitude.
Lilmagoolie said her house "wobbled like crazy" - with the shaking almost as bad as the February shock.
"Big quake. House still moving. Alarms going off everywhere."
Mattdeevee said: "oh my gosh that was so scary!!!!!! Biggest since Feb by far 5.9?!"
Nathanael Boehm, NathanaelB, said he was ready to jump out a
"Was up and ready to leap out a window if it started coming down!"