I’ve spent most of my working life doing drainage and retaining work around the Manawatū, but every so often a job pulls me over the hill into Wellington, and those jobs teach me things flat land never will. One of them was a call from an old workmate who’d retired up to Wadestown, on one of those steep sections where the driveway feels like a ski slope. He rang me in the middle of a wet July because water was sheeting down his bank and pooling against the back of his house. I drove up from Palmy for a look, and it turned into a proper lesson in why hill sections are a different game to our flat Manawatū plots. Down home, water spreads out and sits. On a Wellington hill it moves, fast, and if you haven’t planned for it, it’ll go straight through your house or take your bank with it.
Flat Manawatū plots vs steep Wellington hills
Wellington winters are wet, and the southerlies dump rain in bursts rather than a gentle soak. On a slope that water can’t spread out, so it runs, gathering pace and picking up soil as it goes. Hill suburbs like Wadestown, Khandallah and Karori are full of older villas and bungalows, many going up from the early 1900s through to the 1930s, sitting on banks that were cut and filled long before anyone thought much about subsoil drainage. On my mate’s section the previous owner had done nothing to manage the runoff, and years of it had softened the ground behind his retaining wall.
The real dangers on a hill section, in the order I see them go wrong:
- Water building up behind retaining walls, adding huge pressure until they bulge or fail.
- Saturated ground losing its grip and slipping, which is no joke on a steep bank.
- Runoff pooling against foundations and finding its way inside.
None of that fixes itself. If anything it gets worse every winter until someone deals with it.
What I actually put in the ground
Good hillside drainage is about giving water a planned path off the section before it decides its own path for you. On my mate’s place, and most jobs like it, the toolkit is the same few things used in different combinations:
- Subsoil drains – a perforated pipe laid in a gravel-filled trench – to catch groundwater below the surface.
- A proper drainage layer and weep holes behind any retaining wall so pressure can’t build.
- Surface channels and spoon drains to carry runoff to a safe outlet.
- Catchpits and a connection into the stormwater system where the council allows it.
Behind a retaining wall, drainage isn’t optional – it’s the difference between a wall that lasts and a wall on borrowed time. If you’ve got wall work coming up on a Wellington hill, it’s worth reading our piece on retaining walls on Wellington’s hills alongside this one, since the two jobs go hand in hand.
What it costs, in real numbers
Drainage isn’t the glamorous part of a section, so people put it off – but it’s a lot cheaper than fixing a failed wall or drying out a wet house afterwards. Here’s roughly what I quote, GST included:
| Job | Typical cost (incl. GST) |
|---|---|
| Subsoil drain, dug and laid | $80–$150 per metre |
| Retrofitting drainage behind an existing wall | $3,000–$10,000 |
| Full hillside drainage plan (channels + connections) | $8,000–$25,000 |
Steep, tight Wellington sections sit at the top of those ranges because getting a digger in is half the battle. My honest take: spend on drainage before you spend on anything pretty. A lovely new deck sitting above a failing bank is money down the hill.
When to call someone in vs having a go yourself
On a real hill section, this isn’t always a DIY job, much as it pains me to say it. If the ground is steep or you’ve already seen signs of movement – cracking paths, a wall leaning even slightly, damp patches that won’t dry – get a professional to look before anyone digs. Bad drainage work can make a slope less stable, not more, and that’s a far more expensive mistake to undo than any of the numbers above. It’s also worth knowing that regional rules cover how you manage runoff and where it’s allowed to discharge. The Greater Wellington Regional Council has guidance on stormwater and land stability that’s worth reading before you start moving water around your section.
My mate’s fix wasn’t cheap, but it was a fraction of the cost of rebuilding a collapsed wall, and he’s had dry winters since. That’s really the whole point of doing it properly the first time.
Questions I get asked on hill sites
Why does drainage matter so much more on a Wellington hill section than on flat land? On a slope, heavy winter rain can’t spread out and sit the way it does on flat ground – it runs fast, gathering volume, and pools against retaining walls and foundations. Without planned subsoil drainage and weep holes, that water builds pressure behind walls and saturates the bank, which is what leads to slips and water tracking into the house.
How much does hillside drainage cost in Wellington? As a rough guide including GST: subsoil drains run $80 to $150 per metre, retrofitting drainage behind an existing retaining wall is $3,000 to $10,000, and a full hillside drainage plan with surface channels and stormwater connections is $8,000 to $25,000. Steep, hard-to-access sections push toward the top of those numbers.
Can I fix hillside drainage myself, or does the retaining wall need repairing or replacing? Simple surface channels or spoon drains on a gentle slope can be a reasonable DIY job. But on steep ground, or anywhere you’ve noticed cracking, bulging or movement, get someone qualified to assess it first – retrofitting drainage behind an existing wall is usually cheaper and more sensible than waiting until the wall itself needs full replacement.
What drainage suits older Wellington villas and bungalows on the hills? Older hill properties, many built in the early 1900s through the 1930s, were rarely put in with subsoil drainage at all. On these sites I typically retrofit perforated pipe and gravel-filled trenches, drainage layers with weep holes behind existing walls, and surface spoon drains, rather than trying to force a modern stormwater setup onto a section that was never cut for it. Boundary structures suffer the same way on those sections, and a Wellington fencing contractor will want the water dealt with before setting posts into a wet bank.