‘High and very-high risk’: The Auckland suburbs most vulnerable to landslides
Severe landslides - such as this one at a multi-million dollar property on Auckland's North Shore - are the subject of a major research paper prepared for Auckland Council that looks at likely future slip sites in Auckland. Photo / Jason Dorday
Almost 20% of the Auckland region is at “high” or “very high” risk of being impacted by landslides according to a comprehensive technical report.
That includes some city suburbs, several volcanic cones including Mt Eden and numerous suburbs on the North Shore, including Devonport, Great Barrier Island, Awhitu peninsula and the northern town of Warkworth.
The high-level study carried out for Auckland Council has also highlighted significant landslide risk in Wellsford and at Snells Beach.
The Auckland Region Landslide Susceptibility Assessment technical report says 11% of the Auckland region is at a “high risk” of landslides, and a further 7% “very high risk”.
And the extensive study - prepared with input from council and independent geotechnical and science experts - has also warned climate change will “exacerbate landslide hazards” including from land that hasn’t previously shown weakness.
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.
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.
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.
“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 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.
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”.
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.
Two classes of landslides were mapped:
- smaller scale and shallow, and
- larger 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.
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.
“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.
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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