‘High and very-high risk’: The Auckland suburbs most vulnerable to landslides

For example 103 landslides were reported in the region after the January 15-24 storms this year.

While the study’s mapping has been incorporated into future land use planning, it’s too late for some areas, where houses and communities are already built in high-risk landslide areas.

An aerial view of landslide damage in Muriwai following Cyclone Gabrielle, which is discussed in the new report. Photo / Auckland Council report

Some of these areas are Beach Haven, hilly parts of Devonport and Mt Eden and potentially other volcanic cones, as well as land around Warkworth, Wellsford and Snells Beach.

Auckland Council chief engineer Ross Roberts told the Herald the study will be fundamental to efforts to manage future landslide risk in the city.

But “in many cases the risk is already ‘locked-in’ because of past decisions” he said of homes that were built before slip risk was recognised in their areas.

The study’s detailed information and mapping – delivered to council last May – is available via council’s GeoMaps tool, and also via Land Information Memorandums, also known as LIM reports.

An in-depth Auckland Council report into landslide risks has revealed nearly 20% of the region is at “high” or “very high” risk of being impacted by landslides. Photo / Auckland Council report

Roberts also said council wants to invest in more research and mapping – including potential collaboration with the University of Auckland – because the frequency and severity of high-risk storms is expected to increase.

“We’re developing our work programme and budgets for the next 10 years [with] enhancements in mind,” Roberts said.

The council report shows damage already caused by landslides near dwellings and suggests there will be more to come in future. Photo / Auckland Council report

“Susceptible to rock fall and toppling”

As part of the study, the Auckland region was split into the 11 colour-coded sub-regions seen below.

The area dubbed the Northland Allochthon – which includes Wellsford, Snells Beach and Warkworth – was identified as having significant landslide risk.

That’s because of both natural slope failure and cut slope failure, “large-scale slow moving natural landslides” and slips driven by groundwater.

The Auckland Region was divided into 11 colour coded sub-regions for the study. Photo / Auckland Council report

The presence of excess water in the clay-rich slopes “readily lubricate the movement of landslides” the report says.

The Southern Landslide Zone – east of Manukau – is described as susceptible to landslides after intense rainfall.

Some of these Shore Rd, Remuera homes were red-stickered after extensive flooding and damage in the Auckland region in early 2023. Photo / Brett Phibbs

Islands in the Hauraki Gulf – Great Barrier and Little Barrier Islands are also prone to localised slips during heavy rainfall.

“Large-scale landslides can also occur within the Hauraki Gulf Islands” it says.

By contrast, the composition of other islands such as Waiheke, Kawau, Tiritiri Matangi, Rakino and Motutapu makes them “extremely strong to strong when weathered”.

Islands on the Hauraki Gulf – including Great Barrier Island – have been identified as very prone to landslides. Photo / NZME

The study says the Waitākere Ranges too are “generally competent and stable”.

The Auckland Volcanic Field – comprising around 50 cones, including the popular recreation areas of Mt Eden and One Tree Hill – has some slopes “susceptible to rock fall and toppling”.

Some of the land most at risk of slips and their impacts are the coastal areas around the East Coast Bays and overlooking the inner Waitematā Harbour.

Some land failures “can be exacerbated by earthworks associated with development for infrastructure and subdivisions” the report says.

Landslides are Auckland region’s “most significant natural hazards”

The study’s authors wrote the research could be used to identify areas of the city where “caution should be applied”, and where land should be assessed further.

Their maps highlight areas where landslides are more, and less, likely.

Landslides threaten homes on Colwill Rd, Massey, in early 2023. Photo / NZME

Two classes of landslides were mapped:

smaller scale and shallow, andlarger scale events that “can cause damage across a wider area, often over a longer period”.

The latter could be thousands, or hundreds of thousands of square metres in area.

The two darkest shades of grey highlight areas where the risk is “very high” or “high”. Photo / Auckland Council report

They only show areas where landslides could be caused by storms and flooding, not those triggered by earthquakes.

The 158-page study called landslides “New Zealand’s – and Auckland’s – most significant natural hazards”.

It said Auckland Council seeks to “proactively manage the risk from natural hazards” and work on the region’s resilience.

Roads are vulnerable to landslides, as this photo of damage to the Manukau Heads Rd shows. Photo / Auckland Council report

“As experienced in the 2023 storm events, landslides present a life safety risk, adversely affect property and infrastructure, and may damage or destroy cultural and environmental sites”.

The Auckland region experienced more than 50,000 landslides during and in the aftermath of the Auckland Anniversary floods and Cyclone Gabrielle in early 2023.

Six people lost their lives after last month’s slip at Mt Maunganui, and two more in a second landslide in nearby Pāpāmoa. Photo / Jason Dorday

The January 15-24 storms this year unleashed the deadly landslides that killed eight people in the Bay of Plenty; six holidaymakers at the Mount Maunganui Beachside Holiday Park and a woman and her grandson when their house was struck by another slip at Welcome Bay.

In the aftermath of the Mt Maunganui tragedy, prominent scientist Professor James Renwick told the Herald land use needs to be urgently reviewed in slip-risk areas around the country.

That could include difficult discussions around moving whole communities if they are in harm’s way, he said.

“In most cases, the best way to manage the risk from landslide is to avoid building in areas of higher risk,” the Auckland study’s executive summary said.

“While the risk can be engineered out in some cases, doing this is costly and not always effective.”

Neil Reid is a Napier-based senior reporter who covers general news, features and sport. He joined the Herald in 2014 and has 34 years of newsroom experience.

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