My partner’s family live in St Clair, one of the beach suburbs of Dunedin, and when the sagging retaining wall along their back boundary finally had to go, I got roped into helping them work out what to replace it with. I’m not a tradesperson — I’m a Papamoa first-home buyer who researches everything about my own house the same way: reading, asking tradies slightly dumb questions, and learning from what’s gone wrong on other people’s sections. The question that kept coming back was timber versus concrete, and there’s no single right answer. Here’s what actually helped us decide.
Timber walls: what most older Otago sections already have
Most of the older retaining walls around Dunedin are timber — usually H4 or H5 treated posts with timber rails or sleepers behind them. People build in timber because it’s cheaper up front, quicker to put up, and it suits sloping sites and the look of a typical Otago section. The St Clair section already had a timber wall, so matching what was there felt like the obvious starting point.
Where a timber retaining wall actually gives up
The trade-off is that timber has a use-by date. Even good treated posts rot where they meet the soil once they’re sitting in wet, cold ground for long enough, and Dunedin’s damp winters don’t do them any favours. Realistically you’re looking at 20 to 40 years, depending on the timber grade, the drainage behind it, and how well it was built in the first place. The buried post is the weak point — once that goes, the whole wall goes with it.
Concrete walls: paying more once instead of twice
“Concrete” actually covers a few different builds — poured in-situ walls, reinforced concrete blockwork, and interlocking concrete crib or segmental systems. All three cost more and take longer to build than timber, but done properly they can last 50 to 100 years with almost no maintenance, and they handle damp ground far better.
For the family’s wall, permanence was the deciding factor. They’re not moving, so never having to do this job again was worth paying for. Concrete also carries higher loads more comfortably, which matters on the steeper Otago sites where a wall might be holding back a serious amount of earth.
Timber vs concrete: cost per square metre
These are rough South Island figures including GST — height, site access and drainage all push them around:
| Wall type | Typical cost |
|---|---|
| Treated timber pole wall | $350–$650 per m² of wall face |
| Concrete block wall (footings + reinforcing) | $550–$1,000 per m² |
| Poured or engineered concrete | $700–$1,400 per m², more on hard-access hill sites |
The drainage step nobody mentioned to me at first
Drainage is half the battle whichever material you pick. A wall fails when water builds up behind it, so you’re paying for drainage metal, geotextile and a drainage pipe (novacoil) either way. Skimp on that and even a concrete wall can get pushed out over time.
The consent rule that changes the whole comparison
Here’s the bit that surprised me. Retaining walls over a certain height, or carrying extra load such as a driveway or building above them, generally need building consent and an engineer’s design. In New Zealand, walls up to 1.5 metres that aren’t carrying extra load are often exempt, but anything taller, or on a tricky slope, usually isn’t. The government’s Building Performance site sets out the exemptions clearly, and it’s worth reading before committing to either material — the engineering can change the cost comparison completely. On a hill section especially, sort that out early. If you want the wider cost picture, our guide to the cost of a retaining wall goes through the numbers in more detail.
What we ended up building at St Clair
The family went with concrete block in the end. The wall was over 1.5 metres, so it needed consent and an engineer regardless, and once you’re paying for that, the extra cost of proper St Clair concrete work over timber shrinks a lot. If it had been a low garden wall under a metre, timber would have been the sensible, cheaper call, no argument. That’s the honest answer here: it depends on height, load, how long you’re staying, and your budget. There’s no universally “better” material for a retaining wall — just the better one for your site.
Questions people actually ask us about timber vs concrete walls
Which lasts longer, timber or concrete retaining walls? Concrete, typically 50 to 100 years against 20 to 40 years for treated timber, because timber posts eventually rot where they meet the ground — especially in damp South Island conditions like Dunedin’s.
Do I need consent for a retaining wall in New Zealand? Often, yes. Walls up to 1.5 metres that aren’t carrying extra load are usually exempt, but taller walls, or ones supporting a driveway or building, generally need building consent and an engineer’s design.
Is timber or concrete cheaper for a retaining wall? Timber is cheaper up front — roughly $350 to $650 per square metre versus $550 to $1,400 for concrete. But once a wall needs consent and engineering anyway, that price gap narrows, and concrete’s much longer life can make it the better value call.