Yes, in most rural situations a homeowner or landowner can use a hired mini excavator to install an agricultural (ag) drain on their own property. It's a popular and practical DIY project on New Zealand farms and lifestyle blocks. That said, there are consent requirements, environmental rules, and practical considerations you need to understand before you start digging.
What Is an Agricultural Drain?
An agricultural drain (commonly called an ag drain or French drain) is a subsurface drainage system designed to remove excess groundwater from saturated soil. It typically consists of a trench filled with gravel and a perforated pipe (ag pipe) that collects and redirects water to a suitable outlet — a waterway, stream, or soak pit. They're widely used on lifestyle blocks, farms, and rural sections across New Zealand to manage wet paddocks, boggy ground, and surface flooding.
Is a Building Consent Required?
For most agricultural drainage on rural land, a building consent is not required under Schedule 1 of the Building Act 2004, which lists exempt building work. However, this depends on the scope and location of the work. If your drain:
- Connects to a public stormwater system, you may need council approval.
- Crosses or discharges into a waterway or wetland, you'll likely need a resource consent under the Resource Management Act (RMA).
- Is on land with notable ecological features or near a flood plain, additional restrictions may apply.
Always check with your local council before you dig. Rules vary significantly between regions — what's fine in one district may require consent in another. Contact your regional council (for resource consent matters) and your territorial authority (for building matters). You can find general guidance on consent exemptions at MBIE Building Performance.
Environmental and Regional Rules
This is where many DIYers get caught out. Discharging drainage water into a stream, river, or wetland is regulated under the RMA and your regional council's plan. You must:
- Not discharge sediment-laden water into waterways during or after excavation.
- Check your regional plan for rules around land drainage and waterway setbacks.
- Avoid disturbing wetlands, which are protected under the National Policy Statement for Freshwater Management.
Contact your regional council's environment team early — they're generally helpful and can tell you what's required before you commit to a design.
Choosing the Right Mini Excavator
For most ag drain work on a lifestyle block or small farm, a 1.5 to 3 tonne mini excavator is ideal. These machines are widely available for hire from equipment hire companies around New Zealand. Key considerations:
- Bucket width: A narrow trenching bucket (150–300 mm) is most efficient for ag drain trenches.
- Depth: Most ag drains are installed at 600 mm to 1 m depth, well within the reach of a standard mini excavator.
- Ground conditions: Wet, boggy ground (the very reason you're draining!) can make machine access tricky. Consider tracked machines and plan your access route carefully.
- Spoil management: You'll generate a significant amount of excavated soil — plan where it will go before you start.
How to Install an Ag Drain: Basic Steps
- Plan your layout — identify the wet area, the natural fall of the land, and a suitable outlet point. Water flows downhill, so your drain must have a consistent fall (typically 1:200 minimum gradient, though steeper is better).
- Mark the trench line with pegs and string.
- Excavate the trench to the required depth and gradient using your mini excavator.
- Lay geotextile fabric in the trench to prevent soil migration into the gravel.
- Add a gravel bed (clean, washed chip — typically 20 mm aggregate) to the correct invert level.
- Lay perforated ag pipe (slotted or perforated HDPE pipe, typically 65 mm or 100 mm diameter) on the gravel bed with perforations facing down (note: some installers and manufacturers recommend perforations facing up or to the sides depending on the application — check the pipe manufacturer's installation instructions for the recommended orientation).
- Backfill with gravel to within 150–200 mm of the surface, then wrap the geotextile fabric over the top.
- Backfill the remainder with topsoil and reinstate the surface.
- Protect the outlet with a suitable end cap or screen to prevent animal entry and blockage.
Always check the pipe manufacturer's installation instructions, and source quality materials from a reputable supplier such as Placemakers or Mitre 10.
Safety Considerations
- Dial Before You Dig: Before any excavation, contact Dial Before You Dig (0800 248 344) to identify underground services — power cables, water pipes, and gas lines can all be present even in rural areas.
- Trench safety: Trenches deeper than 1.5 m require specific shoring or battering under WorkSafe NZ guidelines. Most ag drain trenches are shallower, but be aware of soil type — wet, loose, or sandy soils can collapse at lesser depths.
- Machine operation: If you haven't operated an excavator before, consider a brief familiarisation session from the hire company. Never work alone with heavy machinery.
- Overhead lines: Maintain safe distances from any overhead power lines.
When to Call a Professional
While ag drain installation is generally DIY-friendly, consider hiring a qualified excavation contractor or drainlayer if:
- Your drainage connects to a public stormwater or wastewater system (this becomes restricted drainlaying work under the Plumbers, Gasfitters and Drainlayers Board and must be done by a licensed drainlayer).
- The project is large or complex, involving multiple drain runs or pumped systems.
- You're unsure about gradients, soil conditions, or outlet design.
- The ground is very wet or unstable, making safe machine operation difficult.
- A resource consent is required and you need professional support with the application.
You can find experienced local excavation contractors and drainlayers on tradedirectory.co.nz.
Licensed Building Practitioner (LBP)
