'It was Huka Falls in place of our lawn'

Ann and Nigel Nation 's house is teetering over the chasm left by Monday night's slip. Picture / Mark Mitchell

Kelson couple Ann and Nigel Nation yesterday surveyed the wreckage that until Monday evening was their property, and contemplated their lucky escape.

"Just before 9pm there was a huge noise, a sustained roar. I knew something was wrong," Mrs Nation said.

She alerted her husband, who was watching television oblivious to what was happening outside on his property at the top of Speedy's Reserve.

The couple's backyard, once a flat lawn fenced by native bushes, had become a torrent of water.

"It was Huka Falls, 3m to 6m wide in place of our lawn, and the lawn was rushing straight down into the gully," Mrs Nation said.

"Half the lawn had disappeared and there was this incredible earthy smell, wet earth, I couldn't believe how smelly it was. Then more lawn started falling away. I screamed and rang emergency services. And panicked.

"They told us to get out of there, pronto."

The couple bundled Pom-Pom the cat into a cage and fled their house.

The Nations, in their 60s, spent Monday night with family and returned yesterday to find more than 100sq m of land behind their Vista Grove house replaced by a sinking stew of debris, speckled with wooden beams, pipes and mangled trees.

Weeks of rain have left large parts of New Zealand saturated, triggering several slips in residential areas.

The Lower Hutt area has been drenched by 520mm of rain over the past six weeks, including 384mm in July, more than twice the 150mm average.

Institute of Geological and Nuclear Sciences geologist Grant Dellow said the Kelson housing area was a "cut and fill" subdivision, done probably in the 1960s or 1970s when geotechnical practices were not as stringent as they are now.

The Nations' home and a garage and sleep-out at the edge of the slip are likely to fall into a yawning gulf created by the torrent, Hutt City Council spokesman Don Carson said.

The Nations do not know if they will be able to retrieve their belongings.

"Inside is our whole lifetime together, the things of sentimental value, things we treasured as a family together," Mrs Nation said.

"But it makes you understand what's important in life. We're lucky it didn't happen in the middle of the night."

Civil Defence suggested people should regularly inspect and maintain their property and check for signs that the ground may be moving.

Signs included sticking doors and window frames, gaps where frames were not fitting properly, or decks and verandahs that moved or tilted away from the house.

Cracks and bulges in the ground, leaning trees and fences, and moisture in ground that was usually dry were also signs of movement.

"A particularly wet winter has saturated the ground and destabilised some foundations," Air Marshal Ferguson said.

"The most important thing is to be aware of the hazards we face and be prepared for the worst."

Thousands of landslides occur in New Zealand each year, most triggered by heavy rain.

Last year, landslide damage cost the Earthquake Commission $40 million.

This included slips at Matata and Tauranga, triggered by "one in a hundred years" downpours.

Mr Dellow said slips often occurred when land that had been modified was further destabilised by heavy rain.

The land gave way when the "shear stress" measurement of a slope became greater than the "shear strength" that held it together.

When rain falls, the strength of a slope can be weakened by materials within the soil such as clay, which becomes heavier when wet.

Parts of New Zealand have been heavily modified by agriculture, roading and housing.

Slips on farmland are common and are often caused by cutting down trees and other vegetation, the roots of which helped bind the soil.

Flooding yesterday caused disruption in Wanganui and in Christchurch, where the swollen Heathcote River threatened 13 houses.

The worst may be over for waterlogged Wanganui, where a deluge of rain over the previous 24 hours caused flooding and slips that closed more than a dozen roads.

THE OUTLOOK

* Cloudy periods or showers for much of the North Island.

* Snow showers affect the Desert Road.

* Showers in the South Island.

* Light snow down to 400 metres in Canterbury.