Wellington retaining walls get bigger Updated for 2025

A windy, wet, and shaky Wellington

Wellington is built on hills. It also has some of the windiest and wettest weather in the country. Couple this with high earthquake risk, and you have a recipe for soil slips all over town. The 2022 winter was the wettest on record, with rainfall almost twice the average (744mm in Kelburn) since records began nearly a hundred years ago.

What does this mean for you?

We’ve seen a massive increase in landslides, urgent retaining wall repairs, and new retaining wall designs throughout Wellington. In fact, we had new enquiries nearly every day that winter—and those trends are continuing in 2025.

But here’s the key update: the retaining wall designs that used to work may no longer meet the latest standards. Why? Because of a significant change in New Zealand's national guidance, MBIE’s Module 6 update.

MBIE Module 6: Why it matters now more than ever

In November 2021, MBIE (Ministry of Business, Innovation and Employment) updated Module 6: Earthquake-resistant retaining wall design. This update reflected a higher level of expected ground shaking during a design-level earthquake across all regions of New Zealand.

In Wellington specifically, the design earthquake loads on retaining walls increased by an average of 82%. That’s a big jump, and it hasn’t gone away.

What does this mean for 2025?

Designs will need to get bigger, stronger, and smarter

If you’re planning a new retaining wall, or repairing one that’s failed, it needs to be designed for these new loads. That includes not just earthquakes, but also static earth pressures, hydrostatic loads, and temporary construction loads.

All these updates are about making sure your retaining wall actually works in the next big storm, or earthquake.

The technical stuff

We compared some standard retaining wall design cases with the old guidance vs the new to highlight some of the structural changes that can be expected. The retained height of the retaining wall in the example is 2.4m; for easy reference, this is also the general ceiling height of a residential house.

We looked at two common types

• Cantilever timber pole retaining wall

• Cantilever reinforced concrete masonry retaining wall

And all design cases consider the following information:

Click on the images to see a higher resolution detail

Changes in timber pole walls:

For the same retained height, soil properties and pole spacing, you can expect to see:

1. Deeper foundation embedment depth (approximately ~20% deeper)

2. Larger pole size in some cases. (In this case, the bending of the poles was not the governing factor and therefore did not lead to an increase in pole size.)

3. Closer pole spacing in some cases

4. More double lagging at the bottom of the wall

5. Larger diameter foundation holes for poles to achieve the back angle (approximately ~10% larger)

Summarised findings for cantilever concrete block wall:

For the same retained height and soil properties, you can expect to see:

1. Bigger wall section seize (Approximately ~25% bigger)

2. More wall reinforcing in some cases

3. Longer footing length (Approximately ~15% longer)

4. Deeper shear key (Approximately ~60% deeper)

5. Longer heel in some cases (where the overturning governs the design)

6. More steel for the foundation

What can you do?

With hills, wind, and rain, Wellington will always need retaining walls. Add earthquakes to that factor, and you have a recipe for disaster if your retaining wall is not designed correctly. If your retaining wall needs building consent, a Structural or Geotechnical engineer like us can help consult on what type of wall suits your needs and design it to the latest MBIE guidance.

2025 Update: What's changed since the Module 6 Update

It’s been over three years since the MBIE Module 6 update was released, and engineers, councils, and builders have had time to adapt. Here’s what we’ve seen in practice:

1. Councils have become more rigorous in consent reviews

Most Wellington-area councils are now enforcing Module 6 strictly, especially for walls over 1.5m or those supporting driveways, buildings, or slopes near property boundaries.

Expect more RFI (Requests for Information) during consent if your design doesn't clearly reference current seismic loadings or hydrostatic pressures.

2. Costs for retaining walls have increased significantly

Clients are often surprised at the new scale and cost of retaining walls that comply with updated seismic and stormwater guidance. We've seen 15–30% increases in construction costs since pre-2022 designs, sometimes more if access is difficult.

3. Higher scrutiny on older retaining walls

Many existing timber and masonry walls were never designed for earthquake loads. Some Wellington insurers and property managers now require assessments or strengthening as part of property sales or capital upgrade works.

Latest news

Our director, Marlo Bromley, has recently been interviewed about the problems wellingtonians face with old retaining walls. Have a listen here: https://www.rnz.co.nz/national/programmes/morningreport/audio/2018870846/hundreds-of-wellington-retaining-walls-need-fast-attention If you want to learn more about when you need a building consent for your retaining wall, have a read of our article here: https://www.dtce.co.nz/post/when-does-your-retaining-wall-need-building-cons If you want to learn more about what type of retaining wall you can build, have a read of our article here: https://www.dtce.co.nz/post/what-types-of-retaining-walls-can-i-build