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Prime Minister John Key today said an estimated 100,000 homes were damaged by Saturday's 7.1-magnitude earthquake - nearly half of all dwellings in the Canterbury region.

Mr Key made the announcement at a press conference this afternoon, when it was announced the Government will give $5 million to the Mayoral Relief Fund for those affected by the quake.

The 100,000 figure shows the true extent of the quake's damage.

Prime Minister John Key said the Mayoral fund has been set up for those not covered by social welfare. The fund, with the added help of the public and some banks, is now up to $6m.

Mr Key said of the 160,000 homes in the Christchurch, Selwyn and Waimakariri areas, 100,000 could be damaged and some will be beyond repair.

The Government has set aside $94m cost for emergency works when the infrastructure repair and replacement bill for the area is properly estimated.

Speaking after a three hour cabinet meeting this afternoon, Mr Key said he was "awe-struck" by the damage caused to the city where he grew up.

He said many homes and businesses will be covered by insurance and the Earthquake Commission (EQC) but those without insurance will be looked at on a "case by case basis".

"We have to be very careful in that situation. A very strong message needs to be sent to New Zealanders and that is: You need to have insurance," Mr Key said.

Mr Key said the EQC had already received 15,000 claims. He said three field offices will be opened in Christchurch, each with 40 staff.

Senior Cabinet Minister Gerry Brownlee has been appointed Minister for Earthquake Recovery and will be based in Christchurch.

Mr Key said the entire scope of the repair job is still not known.

He said the repairs will likely still be going on a year from now.

He also said he did not have any concerns about infrastructure in the region for next year's Rugby World Cup.

Local government elections in Christchurch would go ahead as scheduled and Mr Key would take his planned trip to Europe on Friday.

Radio station evacuated

This afternoon's 4.5 magnitude aftershock has pushed a number of damaged Christchurch buildings over the edge.

The 12 story Radio Network-Newstalk ZB building in Worcester Street is one that officials have ordered be evacuated after the tremor at 12.35pm.

It had sustained some damage during Saturday's quake, and the aftershock has made some of the cracks worse.

Radio Network stations will continue to broadcast from back up facilities.

Lucky escape

Two people also escaped unharmed after running from an unstable brick building which showed signs of collapsing during the same strong aftershock this afternoon.

The owners of vintage store La Boutique, mother and son Rose and Myles Lennon, were in the brick building on the corner of Westminster and Cranford Streets that houses the shop and other retail outlets when a 4.5 magnitude aftershock hit at around 12.35pm.

Ms Lennon said she was in the shop "grabbing whatever I could" when the shaking began.

"I forgot everything else and just ran for it," she says.

"There were huge cracks down the walls and there was dust and water coming down. The whole place was shaking. It was scary."

Myles says "it was really, really intense. I had a real sense of urgency come over just to get out."

Emergency services were called to the scene and the building, in the suburb of St Albans, is now being destroyed.

Inspector John Price of Christchurch police said safety was paramount and people should steer clear of the building.

"Cordons may vary depending on the weather conditions and possible aftershocks. As civil engineers and USR teams move through the city, further buildings may be deemed to be at risk," he said.

Mr Price also issued a warning to the "small number of people interfering with taped off areas and removing safety cones.

"Removal prevents the protection and safety of everyone."

Aftershocks rock region

The incident followed news of a significant aftershock which reportedly knocked out the lights at Christchurch Airport.

According to GeoNet, there have been 18 significant aftershocks today, ranging in magnitude from 3.5 to 4.8. The latest, at 4.31pm, was a 3.0 magnitude shock centred at a focal depth of 8km depth which struck within 5km of Darfield.

The largest aftershock today was at 4.06am - a 4.8 magnitude, 10km deep quake centred 10km south of Coalgate, a town west of Darfield.

Experts say while aftershocks may continue for weeks, they will diminish in size and frequency and the worst are likely to be over by today.

Demolition begins

Despite the aftershocks, contractors have begun tearing down dangerous buildings half destroyed in Saturday's 7.1 magnitude quake.

Diggers on Victoria and Bealey Street in the city are tearing down what is left of a two storey food market that used to stand opposite Knox Church.

Christchurch's Central Business District is still cordoned off but the extent of the cordon has been reduced overnight.

People are still being asked to stay away while assessments are done.

More than 500 buildings in the city are badly damaged, 90 significantly which may need to be demolished.

It's estimated just under 4,000 homes are without power - 98 per cent of homes have had power restored.

Parts of CBD could be closed for weeks

Authorities say some parts of the Christchurch central business district could be off-limits for weeks until damaged buildings are demolished or declared safe.

Christchurch City Council staff are evaluating damage to buildings and colour-coding them - green for sound, red for dangerous, and yellow to indicate structural problems.

Business owners in the central city will be allowed to check their stores today but will not be allowed to open for business.

Water and sewage a major concern

About 60 streets across Christchurch are still without water this morning as aftershocks continue to shake the city after Saturday's earthquake.

To date, almost a third of all jobs recorded relating to water and wastewater have been resolved and teams are working around the clock to restore water supply to the city.

Christchurch residents are being told to boil their water. KiwiRail has shunted in 14 milk tankers carrying 20,000 litres of water each.

Teams will be working today with cameras to inspect pipes to identify the highest priority repairs.

Christchurch mayor Bob Parker says a number of cases of gastroenteritis have been reported in the city, according to Newstalk ZB.

Sewerage pipes as well as water pipes have been leaking. All schools in the Christchurch, Waimakariri and Selwyn districts are closed until at least Wednesday.

In Rolleston the local council is warning residents that their water supply has been contaminated.

A spokeswoman said an update this afternoon would advise people where to go for water.

She said everyone should be treating their water as contaminated until told otherwise.

"The contamination is due to cracks and leaks in the waste water pipes," she said.

About 180 staff worked on water supply and waste water last night.

0800 helpline

The number of people calling the Government's helpline has more than doubled since yesterday afternoon, Social Development Minister Paula Bennett says.

More than 2500 people have now called the 0800779997 helpline set up to offer advice and referrals for other services.

Just over 1000 had called by yesterday afternoon.

Ms Bennett said five Work and Income offices were confirmed as safe and are open today in Christchurch to provide support.

Four other offices in Canterbury remained closed due to structural damage.

Benefit and superannuation payments will continue without disruption, Ms Bennett said.

* 500+ Buildings damaged
* 90+ CBD buildings damaged
* Estimated cost of damage: $2 billion
* Magnitude of Saturday's quake: 7.1
* 58 Aftershocks
* 245 Residents housed in welfare centres
* 15 per cent of Christchurch homes still without water
* 3500 homes still without power
* 300,000 litres of water transported to Christchurch by rail yesterday
* Distance of epicentre from Christchurch central: 40km

Map: Christchurch earthquake damage
Red marker: Epicentre
Blue markers: General damage
Green markers: Public service advice and information
Yellow markers: Locations of welfare centres
Purple markers: Damage to historic buildings

View Christchurch earthquake in a larger map