Gabion baskets are a popular and practical choice for retaining walls on steep New Zealand hillside sections — they're robust, drain well, and blend naturally into the landscape. The good news is that smaller gabion walls can be a genuine DIY project, but once your site gets steep or your wall gets tall, you'll quickly move into territory that requires engineering input, building consent, and quite possibly a licensed professional.
What Are Gabion Baskets?
Gabion baskets are wire mesh cages filled with rock, stone, or recycled concrete. Stacked and tied together, they form a flexible, permeable retaining wall that handles New Zealand's variable soils and heavy rainfall particularly well. Because they allow water to drain freely through them, they reduce the hydrostatic pressure that causes many solid retaining walls to fail — a real advantage on our wet hillside sections.
When Is a Building Consent Required?
This is the critical question for any retaining wall project. Under Schedule 1 of the Building Act 2004, retaining walls that retain no more than 1.5 metres of ground and do not support any surcharge (such as a driveway, building, or vehicle load) are generally exempt from building consent. However, this exemption has conditions, and your local council may have additional rules — especially on steep or unstable ground.
Always confirm with your local council before you start. MBIE Building Performance has clear guidance on consent exemptions and when engineering is required. On a steep hillside, the risk of slope instability means councils often want to see engineering sign-off even for walls under 1.5 m.
Is a Steep Hillside Section Suitable for DIY Gabions?
Steep sections introduce several complications:
- Slope stability — the hillside itself may be prone to movement, slippage, or erosion, particularly in areas with expansive clays or high rainfall (common across much of NZ).
- Foundation conditions — gabion walls need a firm, level base. On a steep slope, preparing this safely can be challenging and may require excavation.
- Surcharge loads — if there's a house, driveway, or fence above the wall, the wall must be engineered to handle those loads.
- Height and tiering — a single tall wall is riskier than tiered, shorter walls with planting between them. Tiering is generally recommended on steep sites.
- Seismic loading — New Zealand's earthquake risk means retaining walls in many areas must be designed to handle lateral forces. Check BRANZ guidance and your local council's requirements.
As a general rule: if your wall will be under 1 m tall, on stable ground, with no surcharge above it, and your section isn't excessively steep, a careful DIYer can often tackle gabion installation themselves. Beyond that, get a geotechnical or structural engineer involved.
How to Build a Gabion Retaining Wall: Key Steps
1. Plan and Check Consent
Measure the height of ground you need to retain. Contact your council to confirm whether consent is needed. If in doubt, get a Licensed Building Practitioner (LBP) or engineer to assess the site. Find qualified retaining wall builders on tradedirectory.co.nz.
2. Prepare the Foundation
Excavate a level trench at least 200–300 mm deep for the base course. Compact the base well. On sloped ground, step the foundation down the slope rather than trying to create one flat level.
3. Assemble and Fill the Baskets
Gabion baskets typically come flat-packed and are wired together on site. Fill with hard, angular rock (avoid round river stones — they shift). Pack tightly, placing larger stones on the faces for appearance and stability. Use internal tie wires every 300 mm as you fill to prevent the mesh from bowing outward.
4. Tie Courses Together
Each layer of baskets should be wired to the layer below. Stagger the joints like brickwork for strength.
5. Batter the Wall
For stability, lean the wall slightly back into the slope — a batter of around 5–10 degrees is common. This helps the wall resist the pressure of the retained soil.
6. Backfill and Drainage
Backfill behind the wall with free-draining gravel. Good drainage is essential — even though gabions are permeable, saturated soil behind the wall adds enormous pressure.
Materials and Where to Find Them
Gabion baskets and suitable fill rock are available from landscape suppliers, quarries, and some hardware stores. Mitre 10 and Bunnings stock gabion baskets in standard sizes. For larger projects, contact a local quarry for bulk rock supply — this is usually more economical.
| Wall Height | Consent Likely Needed? | Engineering Recommended? |
|---|---|---|
| Under 1.0 m, no surcharge | Usually no (confirm with council) | Not usually |
| 1.0–1.5 m, no surcharge | Check with council | Advisable on steep sites |
| Over 1.5 m | Yes, in most cases | Yes |
| Any height with surcharge | Likely yes | Yes |
When to Call a Professional
You should engage a qualified retaining wall builder or structural/geotechnical engineer if:
- Your wall will exceed 1.5 m in height
- There is any structure, driveway, or vehicle load above the wall
- Your section has a history of slippage, erosion, or unstable soils
- You're in a high-seismic zone
- Your council requires engineering sign-off
- You're unsure about any aspect of the design or ground conditions
Retaining wall failures can be dangerous and costly. A professional assessment is a worthwhile investment. You can find experienced local retaining wall builders on tradedirectory.co.nz.
For work involving drainage connections to public stormwater systems, you'll need a licensed drainlayer — check the Plumbers Gasfitters & Drainlayers Board for licensed practitioners.
Licensed Building Practitioner (LBP)
