A geotechnical expert says some Christchurch homes too dangerous to enter because of the risk of rockfall should not, in hindsight, have been granted building permits.
The Canterbury Earthquake Recovery Authority has been showing media the damage caused by boulders which have torn through houses in the Port Hills in the south of the city.
Hundreds of homes have been written off because of the risk of rockfall and the authority estimates it could take up to three years to finish demolishing the affected houses.
Last week, a large boulder the size of a minivan crashed down a hill and into an abandoned home in Finnsarby Place in the seaside suburb of Sumner.
On a tour of the area on Thursday, CERA chief geotechnical engineer Jan Kupec said it could take decades before some of the properties are safe to build on again.
Mr Kupec said given what experts now know about the risk of earthquakes and the subsequent risk posed by falling boulders, some of the homes should never have been built.
Another geotechnical expert, Camilla Gibbons, said hot weather had caused the rock expand during the day and then contract as the night cooled, and this was enough to cause the boulder to dislodge in Sumner.
Ms Gibbons said the hills are so unstable that boulders could continue to pose a threat to homes for decades to come.
However, one red-zoned Sumner resident, who asked not to be named, believed a number of steps could be taken to prevent boulders from posing a threat, including building large barriers and freeing already loose rocks.
The resident said instead, the authority has decided that such measures would be too expensive.
They believe the Christchurch City Council is being bullied by the Government into giving up on trying to save some homes.