Rock-catching fences could still be used to protect Port Hills properties despite being deemed ineffective for the most at-risk areas.
Nearly 1400 Christchurch homes were last week zoned either red or green, leaving 166 in white-zone limbo until at least August 17, when a decision on landslip or rockfall risk will be made.
Canterbury Earthquake Recovery Authority (Cera) chief geotechnical engineer Jan Kupec said last Friday that fences in most at-risk areas could be ineffective if they were hit "20 to 50 times".
However, Cera chief executive Roger Sutton said yesterday that protective measures for the remaining white zone had not been ruled out.
The red-zoned areas had "vast concentrations" of rocks, which meant the options were unlikely to work or not be cost-effective.
"In the white zone, mitigation is still very much on the table. Fences, bunds and more protection back at the source are still possible," Sutton said.
"In these places where the life risk is less, fewer rocks are coming down and there's a better chance of making [protective measures] work."
Rock-catching fences were used internationally but were uncommon as quake protection.
"With an earthquake, you can have a large number of boulders coming down, the fence gives every time the boulder hits it and eventually the fence is much less effective and can fail," he said.
Moving back into a rockfall area was "all too scary" for some residents, even with protective fencing.
The risk of setting a national precedent for rockfall areas "will have an impact on our thinking", Sutton said.
"We don't want to be doing things that mean in the future I can go and build somewhere with rocks above me and when it turns into a problem the government is going to build me a fence or a bund," he said.
"That is in our mind, but primarily in our mind is the recovery of Canterbury and the recovery of these people."
Sumner resident Phil Elmey, an engineer and builder whose property was white-zoned, yesterday urged Cera to fully investigate area-wide protection before red-zoning more homes.
"We're quite happy in our house and quite happy to resolve this issue collaboratively with the council and the Government and come up with a solution that's acceptable," he said.
Red-zoning homes before attempting to mitigate the risk could not be justified economically, Elmey said.