New Guidance Offers Big Step In Preventing Slope Slip Damage

It will now be easier to identify and evaluate landslides risk thanks to new guidance for geotechnicians from the New Zealand Geotechnical Society (NZGS).

Landslides are New Zealand’s deadliest and costliest natural hazard on average but, until now, there hasn’t been a consistent approach for evaluating landslide hazards.

“The lack of a good practise document can sometimes lead to the same slope being evaluated by different people with completely different results,” says Richard Justice, the lead author of the guidance and an expert in geotechnical risk assessment.

“For years, New Zealand’s geotechnical community has been calling for clear and consistent advice on how to conduct site investigations and hazard assessments for landslides, and we’ve made a big step in that direction through this new guidance.

“This guidance reflects the huge amount of knowledge and research in New Zealand and internationally ranging from land use planning, investigation, model development, stability assessment and mitigation.

“We have pulled together the best of what’s currently available and added our own insights as experienced practitioners,” he says.

The NZGS has this week released the first part of its Slope Stability Guidance, which outlines the best practise on identifying landslide hazard and recommendations on what to do if a landslide or unstable land is identified.

Subsequent units, which will be released throughout 2025, will provide more details on recognising landslides, field investigations, analysing unstable slopes and mitigation strategies.

Richard adds the guidance was a big team effort and he’s grateful to everyone who contributed. “We have included feedback from many organisations and individuals within New Zealand and from as far away as the UK. I’d particularly like to acknowledge the input of our immediate past chairs, Ross Roberts and Eleni Gkeli, who were instrumental in laying the foundations for the project during their tenures.”

Richard hopes the guidance can support practitioners but cautions that it does not replace the need for technical expertise or location-specific advice.

“We will always need to rely on our geotechnical engineers and engineering geologists. The guidance we have developed is meant to help them in their assessment of landslides – but it cannot replace experience and expertise.”

The guidance, which was funded by the Natural Hazards Commission (NHC) Toka Tū Ake with additional support from MBIE, aligns with NHC’s goal of supporting stronger homes built on better land.

Dr Wendy Saunders, NHC’s principal advisor and land-use planning champion, says, “many of us live on slopes, especially in cities where space is limited. This can be a problem after an earthquake or heavy rainfall, because if slopes become unstable and slip, they can cause damage to property and infrastructure. With an increase in both urban development and extreme weather events, we can expect to see more landslips if nothing is done to mitigate risks.”

NHC currently receives more claims for damage due to landslips than any other natural hazard. Since the Auckland anniversary floods in January 2023, losses from landslips have totalled $302 million – over four times the cost from all other natural hazards combined.

“Apart from the financial impact, landslips can be distressing for the people and communities affected. If you live on a slope, there are things you can do to identify signs of potential slipping and prepare your property, like building retaining walls, cleaning your gutters and seeking advice from a geotechnician. Check out our website for more information.

“If you’re buying a new property, you can visit our Natural Hazards Portal for information on past claims related to landslides and other natural hazards, which could indicate future risk,” Dr Saunders adds.

About the Natural Hazards Commission

The Natural Hazards Commission Toka Tū Ake exists to help New Zealanders prepare for and recover from the impact of natural hazards. One of the ways we do this is by providing natural hazards insurance for homes and residential land. Homeowners have access to our scheme if they have an insurance policy with fire cover. The premium paid to insurers includes a Natural Hazards Insurance Levy, which is the amount homeowners pay for this cover.

You can find out more about Natural Hazards Cover/NHCover on our website to inform reporting on the issue. You can also read more about our annual investment in research and resilience through regular updates on our news page.

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