DESTRUCTIVE FORCE: The 40-tonne boulder that came to rest under a Sumner house after crashing down the hillside.

A 40-tonne boulder has been turned into a political football after it smashed into an unoccupied Sumner house.

Canterbury Earthquake Recovery Minister Gerry Brownlee said the rockfall, which may have been caused by the recent dry weather, vindicated the Government's decision to take no risks when it came to red-zoning some Port Hills properties.

But some residents forced out of their homes by the Government's zoning decisions still believe rock protection work is possible.

The van-sized boulder smashed through the deck into poles supporting the red-stickered house at 31 Finnsarby Pl on Wednesday night.

"We know from the extensive ground-truthing and area-wide modelling that the risk of rock roll in this part of the Port Hills is high; hence the need to zone the land red," Brownlee said yesterday.

He said reports from the Canterbury Earthquake Recovery Authority's geologists indicated such rock-roll activity was to be expected, even without an obvious trigger like heavy rain or further earthquakes.

"This boulder is the size of a van, so clearly it posed a risk to life and limb had that home been occupied,'' he said.

''All areas of Port Hills land zoned red due to rock roll are as dangerous, and this cannot be forgotten."

Sumner resident Phil Elmey, who has vowed to fight the red-zoning of his land, said the Finnsarby Pl house was in a "bowling alley".

He said most of the red-stickered houses could be saved if money was spent on rock protection work.

"Even a rock that size could be stopped by the right protection,'' he said. ''We think it's disgraceful that it hasn't happened."

Terry Huggins, who lives opposite the house, said he heard the boulder fall.

Christchurch City Council building operations manager Ethan Stetson said the recent hot weather could have forced moisture out of the soil, causing the ground to contract and the boulder to fall.

"You can have rock movement in any or all of those locations at any time. It doesn't take an earthquake to make things move."

He said the property had been issued a section 124 notice, which barred entry to homes because of the danger of rockfall or landslip.

It was unlikely any more properties in the street would be red-stickered.

"If I had advice from the geotech [people], then that would be considered, but I'm not expecting that," he said.