In most situations, yes — installing agricultural drain coil (also called ag drain or slotted coil) in a paddock is work a capable homeowner or farmer can tackle themselves. It doesn't typically involve the building drainage or sanitary systems that require a licensed drainlayer. That said, getting the design, depth, and fall right is critical, and there are some situations where professional help is strongly recommended.
What Is Agricultural Drain Coil?
Agricultural drain coil is a flexible, perforated plastic pipe — usually 65 mm or 100 mm in diameter — designed to collect excess groundwater from saturated soil and carry it away to a suitable outlet. It's widely available at Mitre 10, Bunnings, and PlaceMakers in New Zealand, typically sold in rolls of 50–100 metres.
The pipe is laid in trenches, usually surrounded by gravel or wrapped in a geotextile sock to prevent fine soil particles from blocking the slots.
Is This Restricted Work Under NZ Law?
This is an important question. Under the Plumbers, Gasfitters, and Drainlayers Act 2006, drainlaying is restricted work that must be carried out by (or under the supervision of) a licensed drainlayer. However, this restriction applies to sanitary drains and stormwater drains connected to buildings or public infrastructure — not to agricultural sub-surface drainage in a paddock.
Installing ag drain coil in a rural paddock to manage groundwater is generally not restricted drainlaying work, meaning a homeowner or farmer can legally do it themselves. However, if your drain outlet connects to a public waterway, road culvert, or council stormwater system, you may need to check with your local council first, as resource consent or approval could be required.
For clarity on what counts as restricted work, refer to the Plumbers Gasfitters & Drainlayers Board.
Planning Your Drainage System
Good planning is the difference between a drain that works for decades and one that fails within a season.
Identify the Problem Area
- Walk the paddock after heavy rain and mark where water pools or the ground stays boggy longest.
- Consider whether the issue is surface water (runoff) or sub-surface water (a high water table or impermeable clay layer).
Design the Layout
Common layouts include:
- Herringbone pattern — lateral drains feeding into a central main drain. Good for large, uniformly wet areas.
- Grid pattern — parallel drains at regular spacing. Suits flatter paddocks.
- Single interceptor drain — a single drain running across a slope to intercept water flowing downhill. Useful for wet seeps.
Spacing between laterals typically ranges from 5 to 15 metres, depending on soil type. Heavy clay soils need closer spacing; sandy or loamy soils can be spaced wider. If you're unsure, your regional council's rural advisory team or a local drainage contractor can help with design.
Outlet Location
Your drain must outlet somewhere water can freely discharge — a farm drain, stream, or purpose-built sump. The outlet must remain unobstructed and free-flowing at all times. Check with your regional council whether discharging to a waterway requires a resource consent under the Resource Management Act 1991.
Tools and Materials You'll Need
| Item | Notes |
|---|---|
| Ag drain coil (65 mm or 100 mm) | Choose size based on catchment area |
| Drainage aggregate / chip | 20 mm clean chip is typical |
| Geotextile filter sock or fabric | Prevents silt ingress |
| Trenching spade or mechanical trencher | Hire a trencher for long runs |
| Laser level or builder's level | Critical for achieving correct fall |
| Outlet pipe and end cap/screen | Keeps vermin out |
For longer runs, hiring a ride-on trenching machine from a local hire company will save significant time and effort.
Key Installation Steps
- Mark out your drain lines with string or marker paint.
- Dig trenches to the required depth — typically 450 mm to 900 mm deep, depending on soil conditions and the depth of the impermeable layer.
- Achieve a consistent fall — a minimum grade of 1:200 (0.5%) is generally recommended to keep water moving. Use a laser level to check.
- Lay a bed of drainage chip (about 100 mm) in the trench bottom.
- Lay the ag coil with the slots facing down or to the sides — not upward, as this can allow sediment in.
- Surround the pipe with drainage chip to at least 150 mm above the pipe, then cover with geotextile fabric before backfilling with soil.
- Connect laterals to the main drain using appropriate junctions and fittings.
- Install and protect the outlet — use a mesh cap to keep out rodents and debris.
Common Mistakes to Avoid
- Insufficient fall — water won't move and the drain becomes useless.
- No filter sock or chip — fine soil particles block the slots quickly.
- Outlet too high — the drain can't empty properly.
- Ignoring the impermeable layer — if you don't get below the clay pan, the drain won't intercept the water table.
When to Call a Professional
Consider hiring a qualified drainage contractor or licensed drainlayer (find one at tradedirectory.co.nz) if:
- Your paddock is large or the drainage problem is complex.
- You need to connect to a public drain, road culvert, or council infrastructure — this will require a licensed drainlayer.
- You're unsure about the correct depth, layout, or fall.
- The work involves any building drainage or sanitary connections.
- You need resource consent advice for discharging to a waterway.
A professional can also use GPS-guided trenching equipment to achieve precise grades across large areas, saving you time and rework.
Safety Note
Trenching work carries real risks. Never enter an unsupported trench without appropriate shoring or battering the sides — check current WorkSafe NZ guidelines for the specific depth thresholds and requirements that apply. Follow WorkSafe NZ guidelines for excavation safety, and always dial Before U Dig (0800 248 344) to check for underground services before you start digging.
Certifying Drainlayer
