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Developers went ahead and built on areas of land that could turn into murky soup after an earthquake, despite legal opposition from the local council, the mayor of Christchurch says.

Christchurch City Council mayor Bob Parker said for the past 30 years his council has insisted Land Information reports record if homes are built on sandy soil deposits and subject to liquefaction.

Whole streets in Christchurch have been built on sandy soil deposits. Shortly before Saturday's magnitude 7.1 quake, rains had caused the soil to take on large amounts of water. The soil was then turned to mush when the ground began to shake - this process is known as liquefaction.

"With the benefit of hindsight, you would probably have to say that the nature of the foundations were not designed to cope in the end with the type of earthquake that we had," said Mr Parker.

"In a number of cases, there are areas where council has indicated that it did not want residential development to go ahead. In a number of cases this has resulted in court cases and on some occasions the council has lost those court cases," Mr Parker said.

He said there have also been some older areas of Christchurch which have suffered liquefaction damage but were developed long before the phenomenon was known.

"But it is interesting to note that the new Deans Stand built in AMI Stadium was built on a liquefaction zone. There is not an iota of damage, settling, or shifting that has occurred in that massive structure. And the reason is that the foundations have been designed to cope with the ground situation," Mr Parker said.

He said building is possible in liquefaction areas but, like the Deans stand, the appropriate type of foundation is needed.

Mr Parker said a lot will be learned about liquefaction from the earthquake in Christchurch.

Building requirements have been changed by the council as a result of the quake.

An urgent change has been pushed through by the council requiring earthquake-damaged buildings undergoing repairs to meet a higher standard of structural safety.

In an extraordinary meeting this morning, the council resolved to adopt a new policy whereby earthquake-prone buildings will have to aim for a goal of 67 per cent of building code levels rather than the existing 33 per cent.

The new standard would bring older buildings up from about 10 per cent of the strength of a brand new building, to about 50 per cent.

The policy comes six days after Canterbury was hit by a 7.1 magnitude earthquake that caused extensive disruption and damage across the region.

"What we are trying to do is make sure buildings don't fall on people," said Christchurch City Councillor Sue Wells.

"What we have learnt through the last little while is that buildings which are strengthened to 33 per cent of (building) code will not provide the security that we are needing."

Cr Wells said the increased costs of meeting the higher building standard would depend on a case-by-case basis.

"The cost to building owners of doing repairs will be vastly less than the cost to them of losing their livelihood, of losing their building, of losing their trading. That is what is happening out there."

Mr Parker said: "It is true to say this will cost more money in some cases."

Asked if the higher standards expected could encourage building owners to demolish rather than rebuild, Cr Wells said the previous standard was not adequate.

"And no-one is going to thank us for letting building owners repair their buildings to an inadequate level of code. Nobody wants to see buildings strengthened just to fall down again."

The council could not delay this change but building owners were already seeking to lodge applications for consent to rebuild.

Mr Parker said the city had a "duty of care" to the people of the city, and the council had to react to the quake.

"It will cause some issues for some property owners. And in the months to come, some people will have to make hard decisions.

"What we will not compromise on in is the safety of our citizens. We are making a decision that will save lives in the future."