Landslides in Wellington: What Causes Them and How to Prevent Damage

Lee Huang - Director

If you live in Wellington, you probably don’t need reminding that our hills are beautiful but also a bit unstable. The steep terrain, combined with heavy rain and earthquake activity, makes landslides (sometimes called landslips) a regular feature of life here. They can block roads, damage homes, and leave property owners with costly repairs. So why do landslips happen, and what can you do to reduce the risk to your home or development project? Let’s break it down.

What causes a landslide in Wellington?

Several factors can trigger slope failure, often acting together:

1. Steep slopes and weak soils - Much of Wellington is built on steep hillsides with soils made of weathered greywacke, colluvium (loose material at the bottom of slopes), or soft fill. These soils don’t always hold together well under stress.
   

2. Heavy rainfall - Intense storms saturate the ground, reducing soil strength and increasing the weight of the slope. The “wet sponge” effect is often enough to push a marginally stable slope into failure.
   

3. Poor drainage - Leaking stormwater pipes, blocked drains, or poorly designed retaining walls can concentrate water into the wrong spot, undercutting slopes and triggering movement.
   

4. Vegetation removal - Tree roots and plant cover help reinforce slopes. When vegetation is stripped away—say, for a new build or driveway—the slope is more vulnerable.
   

5. Earthquakes - Wellington is an active seismic zone. Shaking can weaken soils, dislodge boulders, and in some cases cause whole slopes to collapse.

These same conditions affect coastal and hillside communities across New Zealand, particularly in Picton and the Marlborough Sounds, where heavy rainfall and weak coastal soils make slopes especially vulnerable.

Warning Signs of Slope Instability

If you’re a homeowner or property developer, some early red flags to look out for include:

• Cracks in the ground, driveway, or retaining walls

• Tilting fences or trees leaning downslope

• Slumping or bulging ground at the base of a slope

• New springs or wet patches appearing after rain

• Doors or windows in your house sticking due to subtle ground movement

Tilting fence above a house being propped temporarily

The Natural Hazards Commision (formerly EQC)

The Natural Hazards Commission – Toka Tū Ake (NHC), formerly EQC, provides insurance support for natural disasters, including landslips. Importantly, NHC is one of the only organisations that offers limited cover for land damage—such as slips affecting your section, accessway, or retaining walls that protect your home.

If a landslip occurs on your property:

1. Call your private insurer first. They will manage the claim and coordinate with the NHC behind the scenes.

2. Document the damage. Take photos, note the timing, and keep records of any safety concerns.

3. Don’t try major repairs yourself. Wait for guidance, as remedial work may affect your insurance claim.

Engineers like us are usually involved only once insurers and NHC have assessed the slip and need technical input for design or remediation.

How to Reduce the Risk of Damage

You can’t control Wellington’s rainfall or earthquakes, but there are practical steps that help manage landslip risks.

1. Get a Geotechnical Report - A qualified geotechnical engineer in Wellington can assess the ground conditions, slope stability, and drainage around your property. For building consents, the council will often require a geotechnical report anyway, but even outside of new builds it can save you money (and headaches) in the long run.
   

2. Improve Drainage

   • Install subsoil drains to redirect water away from vulnerable areas

   • Regularly maintain downpipes, gutters, and stormwater systems

   • Avoid discharging water over slopes
     

3. Retaining Wall Design - Retaining walls need proper engineering to manage loads and drain water. A poorly designed or DIY retaining wall is often a landslip waiting to happen. Further watching: If you'd like a clear visual explainer on different types of retaining walls and why they sometimes fail, this video from Practical Engineering is excellent. It covers the core principles in an easy-to-understand way. For Wellington and Malborough-specific design, though, you'll still need advice tailored to your site conditions - and that's where we come in.
   

4. Planting and Vegetation - Native deep-rooted plants (like flax or cabbage trees) help stabilise soil, while grass cover reduces surface erosion. Strategic planting is a low-cost way to add slope stability.
   

5. Regular Monitoring - After major storms or earthquakes, check your property for new cracks, water seepage, or ground movement. Early action is often cheaper than post-failure repairs.
   

The cut before a retaining wall is built. In Wellington retaining walls serve as a major structure to protect a house from landslips

The Technical Jargon: How Engineers Assess Slope Stability

For those who like the nitty-gritty, here’s what geotechnical engineers look at when assessing whether a slope is stable:

• Factor of Safety (FoS): This is the ratio of resisting forces (soil strength, friction, reinforcement) to driving forces (gravity, water pressure). If FoS > 1, the slope should be stable. For residential projects, engineers typically aim for an FoS between 1.3 and 1.5.
  

• Shear Strength Parameters: Soils are tested for cohesion (c) and angle of internal friction (φ), which determine how well they resist sliding. Soft clays tend to have low shear strength, while gravels are much stronger.
  

• Pore Water Pressure: Excess water in the soil reduces effective stress, which reduces shear strength. That’s why drainage is such a big deal.
  

• Critical Slip Surfaces: Engineers model potential failure surfaces through the slope—using methods like limit equilibrium or finite element analysis—to see if and where movement might occur.
  

• Seismic Loading: In Wellington, we also apply earthquake coefficients in stability analysis, checking whether a slope would hold up under strong shaking.

Why Engage a Geotechnical Engineer?

At DTCE, our geotechnical engineering team specialises in slope stability analysis, site investigations, and foundation design. We prepare geotechnical reports for Wellington property owners, architects, and developers that not only meet council requirements but also give you clear advice on how to protect your site.

We have also worked closely with insurers and the Natural Hazards Commission to assess slope failures and damage after landslips, providing the technical input needed to guide repairs or rebuilds.

While much of our work is in Wellington, we regularly assist in other high-risk areas such as Picton and the Marlborough Sounds, where steep coastal slopes and heavy rainfall make landslips a common issue. Our experience in these regions means we can provide practical, localised solutions that stand up to the environment.

Email us now!

Or call us on on 04 475 7933

About the Author

Lee Huang is a Director at DTCE and a Chartered Professional Engineer with over 15 years of experience in New Zealand. He leads our ongoing effort to expand our Geotechnical capabilities alongside our Senior Engineer, Kyle Nicholson