Auckland stores up trouble

An illegal stormwater pipe may complicate an insurance claim for a million-dollar home teetering on the edge of a cliff but hundreds of Auckland homeowners could wind up with the same problem.

Heavy rain caused a landslip below the $1.2 million Birkenhead Point home this week, taking a deck post with it and leaving the house, on a 40m-high cliff, uninhabitable.

Earthquake Commission insurance manager Lance Dixon would not comment on the Awanui St house, saying its UK-based owners arrived in the country only yesterday.

"It will depend on what advice our engineers give," he said.

North Shore City spokesman Paul O'Brien said an illegal drain might have contributed to the landslide but it was "one of a number of potential factors".

"We do have problems on a wider scale of people discharging water on to a cliff face which, if you live on top of it, is the height of stupidity," he said.

A wander around Auckland's beachfront shows hundreds of white PVC pipes poking out from banks and cliffs below Parnell, Mission Bay, Bayswater, Takapuna Beach, Cheltenham and Narrowneck Beach.

While water discharged at the wrong place undermines cliffs, they are also eroding naturally.

Tonkin Taylor engineering consultant Richard Reinen-Hamill said the cliffs around the Waitemata eroded at a rate of about 1m to 3m every 100 years. Around the Manukau, it was worse with a 1m to 10m rate.

"Managing stormwater and vegetation is slowing the process but won't stop it," he said.

"You get slips in particular locations and those can be human-triggered even if it's something that is eventually going to happen."

Professor Michael Crozier, of Victoria University's School of Geography, Environment and Earth Sciences, said this country was a geological youngster, meaning volcanoes still erupted and land was still moving.

A landslide in Tauranga last year and another on Wellington's Oriental Parade earlier this year had occurred because those areas were once beaches.

"There was water lapping at the base of where the homes are in Oriental Parade in 1855 so that cliff is still adjusting," he said.

"Cliffs can adjust for hundreds of years and that was a factor in those situations."

Greg Paterson, water planning manager for Auckland City, said problems existed around coastal cliffs where no stormwater system was in place.

"The easiest place in that case is to discharge over the cliff," he said.

But stormwater upgrades were "a matter of priorities".

Under its long-term plan, the council has raised expenditure on the stormwater system from about $23 million a year to $30 million annually for the next six years.

North Shore City needs to spend $140 million over the next decade.

Auckland Regional councillor and North Shore resident Joel Cayford said homeowners in potentially hazardous zones should be forced to connect up to the stormwater system even if it meant a $50,000 bill for new pipes leading away from cliff edges. "If property owners don't take care of stormwater, they are putting those cliffs at risk," he said.


Risky land

Areas prone to landslips include:

Auckland: 825ha of land, mainly in Mt Roskill, Pt Chevalier, Otahuhu, Glendowie, Avondale, Hillsborough and Orakei, affecting 1812 homes and 30 businesses.

North Shore: No landslip danger areas but testing of areas suspected of being unstable.

Waitakere: Several thousand households in Titirangi, Laingholm and Glen Eden.

Manukau City Council: Coastal areas such as Orere Pt and Bucklands Beach, affecting about 200 houses.

Tauranga: About 200 properties in the older pre-1995 part of the city on steep slopes.

Gisborne: About 30 to 40 houses on city hills.

Wellington: Hazard areas marked for earthquakes (Thorndon) and floods (Tawa) but not slips as there has been no historical need.