The leach drain (or absorption trench) is the final stage of a conventional septic system, it receives the clarified primary effluent from the septic tank and distributes it through the soil for natural treatment and dispersal. Get the sizing wrong and the system will overload: effluent will surface in the yard, back up into the house, or leach untreated waste toward groundwater or waterways.
In NSW, leach drain sizing is governed by AS/NZS 1547:2012 (Onsite domestic wastewater management) and must be based on site-specific data from a soil and site assessment. This guide explains what that assessment involves, how the calculations work, and what Port Macquarie Hastings Council expects to see in a DA submission.
Why Sizing Matters More Than Many People Realise
A common assumption is that septic systems are simple, dig some trenches, lay some pipe, connect the tank. In reality, the land application area is the most technically demanding part of the system. An undersized absorption area will fail within a few years regardless of how well the tank is maintained. An oversized area is unnecessary expense. Correct sizing requires real data from your specific site.
Step 1: Determine the Design Wastewater Flow
Before you can size the absorption area, you need to know how much effluent the system must handle daily. In NSW residential applications, the standard design figures are:
| Bedrooms | Design Occupancy | Design Wastewater Flow |
|---|---|---|
| 1 | 2 persons | 240 L/day |
| 2 | 4 persons | 480 L/day |
| 3 | 6 persons | 720 L/day |
| 4 | 8 persons | 960 L/day |
| 5 | 10 persons | 1,200 L/day |
Note: These are design figures, not what people actually use. The 120 L/person/day figure in AS/NZS 1547:2012 is conservative to account for peak loads, laundry days, and guests.
For non-residential applications (commercial, holiday rentals with high peak occupancy, farm worker quarters), the design load calculation must reflect actual expected usage patterns, not bedroom count alone.
Step 2: Soil and Site Assessment
The soil assessment determines the hydraulic conductivity of the soil, how fast water moves through it. This is the core variable that determines how much trench length (or trench area) you need.
Soil Classification
AS/NZS 1547:2012 classifies soils into categories based on texture, structure, and percolation rate:
| Soil Class | Description | Long-Term Acceptance Rate |
|---|---|---|
| Class 1 | Sandy, coarse, rapid drainage | High rate, BUT may require treatment for pathogen protection |
| Class 2 | Sandy loam, moderate drainage | Moderate-high rate, suitable for conventional systems |
| Class 3 | Loam, clay loam, moderate drainage | Moderate rate, commonly used for sizing |
| Class 4 | Clay, slow drainage | Low rate, larger area required |
| Class 5 | Heavy clay, very slow drainage | Very low rate; conventional absorption may not be feasible |
Most rural properties around Comboyne (basalt soils) fall in Class 2-3. Sandy coastal soils near Lake Cathie can be Class 1 (requiring treatment upgrades despite fast drainage). Heavy clay areas on Wauchope’s lower floodplain or some King Creek blocks can be Class 4-5.
Percolation Test
The percolation test (perc test) measures how fast water moves through the in-situ soil at the proposed absorption area depth. It involves:
- Excavating or boring test holes to the proposed trench depth (typically 400-600 mm)
- Pre-soaking the holes for 12-24 hours to simulate saturated field conditions
- Measuring the rate at which a standard depth of water falls over timed intervals
- Calculating the percolation rate in minutes per millimetre (mm/min) or similar units
The perc test should be conducted at multiple locations across the proposed absorption area to account for soil variability. Seasonal timing matters too, a perc test in summer dry conditions may overstate drainage capacity compared to winter saturated conditions. Consultants working in the Port Macquarie region will account for the seasonal rainfall patterns of the Mid North Coast.
Step 3: Calculate Required Trench Area
Using the design flow and the soil’s long-term acceptance rate (LTAR), the required absorption area is calculated:
Required Area = Design Flow ÷ LTAR
The LTAR is derived from the percolation test results, typically expressed in litres per square metre per day (L/m²/day).
Example calculation:
- 3-bedroom house = 720 L/day design flow
- Soil LTAR from perc test = 20 L/m²/day (moderate clay-loam, Class 3)
- Required area = 720 ÷ 20 = 36 m² of trench base
In a standard 500 mm wide absorption trench, 36 m² of base requires approximately 72 m of trench length. Add a 100% reserve area (required by AS/NZS 1547:2012) for future use if the primary area fails, and the total trench system runs to approximately 140-150 linear metres.
Minimum Setback Distances for Leach Drains
Sizing calculations tell you how big the absorption area needs to be. Setback distances tell you where it can go. All of these must be satisfied simultaneously:
| Potential Conflict | Minimum Setback from Absorption Area |
|---|---|
| Dwelling | 15 m |
| Property boundary | 3 m |
| Waterway (creek, river, dam, wetland) | 50 m (conventional system) |
| Water supply bore (own property) | 100 m |
| Water supply bore (adjacent property) | 100 m |
| Roads, driveways | 6 m |
| Trees (canopy) | 3 m |
| Cut or fill batters | 15 m |
On smaller rural-residential blocks around Wauchope and Beechwood, these setback requirements can significantly constrain where the absorption area can physically go. If setbacks limit the available area to less than what sizing calculations require, an AWTS with drip irrigation (which has much smaller setback requirements) becomes the practical solution.
How Many Trenches? Trench Geometry
Absorption trenches are typically constructed as:
- 500 mm wide
- 400-600 mm deep to the perforated pipe (gravel bed below pipe)
- Minimum 600 mm separation between parallel trenches (often 1.5-2 m)
- Maximum 30 m per trench run (longer trenches risk uneven loading)
A 150 m total system might be designed as five 30 m trenches running parallel down a slope (always across the contour, never down the slope, to prevent channelling and ensure even loading).
Trenches must run approximately on-contour so effluent spreads evenly. A slope of 1-5% is ideal. Steeper slopes (>10%) make even distribution difficult and may require modifications such as stepped trenches or alternative dispersal methods.
When Leach Drains Are Not Suitable
Conventional leach drains are not suitable where:
- The soil LTAR is too low (heavy clay, the area required becomes impractically large)
- The seasonal water table is within 600 mm of the surface (trenches will flood)
- Site constraints prevent placement outside required setback distances
- The site is in a flood hazard area that regularly inundates
- A high water table in coastal areas (Lake Cathie, North Haven) means trenches periodically saturate
In these cases, the alternatives are:
- AWTS with drip irrigation (see our AWTS guide)
- Mound systems (raised absorption area above natural ground level)
- Biofilter with subsurface drip irrigation
- In extreme cases, a holding tank
Council Documentation Requirements
When lodging a DA for a new septic system with Port Macquarie Hastings Council, the absorption area design must be supported by:
- Soil assessment report from a qualified consultant
- Percolation test results and methodology
- Hydraulic loading calculations showing design flow, LTAR, and required area
- Scaled site plan showing trench layout with dimensions and setbacks
- Statement that the design complies with AS/NZS 1547:2012
Council’s health and environment officers will check that the calculations support the proposed area and that all setbacks are satisfied. Under-documented submissions are a common cause of DA delays.
FAQ
Can I design my own leach drain? In NSW, the septic system design and site assessment must be carried out by a suitably qualified person, typically a hydraulic engineer or environmental consultant, and the physical installation must be by a licensed plumber and drainer. Homeowners cannot self-certify the design.
My old leach drain is failing, do I need to redo the entire system? Not necessarily. If the tank is sound and the failure is isolated to the absorption area, the failed trenches can be decommissioned (backfilled) and new trenches installed in the reserve area. If no reserve area exists, you may need to locate additional area within the setback constraints. A septic system inspection will assess what is salvageable.
What does it cost to install a leach drain system near Wauchope? Costs depend heavily on soil conditions, site access, total trench length, and materials. For a typical 3-bedroom rural property near Wauchope, a septic tank plus absorption trench system installed runs $8,000 to $15,000 in 2026. See our full installation cost guide for a detailed breakdown.
Why do I need a 100% reserve area? The reserve area is a precautionary requirement in AS/NZS 1547:2012. Over 20-30 years, the primary absorption area accumulates biomat that gradually reduces its permeability. Having a reserve area means the system’s effective life can be extended by switching to the reserve when the primary area is exhausted, rather than requiring a full new installation.
My percolation test showed Class 1 sandy soil, does that mean I need a smaller leach drain? Not quite. Very fast percolation (Class 1) does allow more loading per square metre, but AS/NZS 1547:2012 caps the design loading rate for fast soils to prevent pathogen breakthrough. A very fast-draining sandy soil may require an enhanced treatment system (AWTS or biofilter) before land application precisely because effluent passes through too quickly for natural pathogen attenuation.
Need help understanding your septic system’s land application area or concerned that an old trench is failing? Send an enquiry for an inspection and assessment.