Before any new septic system can be designed and approved in NSW, the site must be assessed and the soil tested for its ability to accept and treat effluent. This process, combining a physical soil profile assessment with a percolation (perc) test, is not a formality. It is the technical foundation on which the entire system design rests. An inaccurate or incomplete site assessment produces a system design that may fail within years of installation.
On the Mid North Coast and Hastings region, where soil types vary enormously across short distances, from volcanic basalt loams at Comboyne to coastal sands at Lake Cathie to alluvial clay on the Hastings floodplain, site assessment results cannot be predicted by location alone. Every individual site needs its own assessment.
What a Site Assessment Covers
A comprehensive soil and site assessment for onsite sewage management in NSW as required by AS/NZS 1547:2012 includes:
1. Site Description
- Land use history and any fill or disturbed soil areas
- Topography (slope, drainage direction, cut and fill areas)
- Proximity to waterways, dams, bores, and drainage lines
- Proximity to property boundaries, dwelling, and other structures
- Flood risk assessment (particularly important on the Hastings floodplain)
- Vegetation type and root depth considerations
2. Soil Profile Assessment Pits or test holes (typically 1.2-1.8 m deep) are excavated at multiple locations across the proposed land application area. The soil profile is described at each depth interval:
- Texture (sand, loam, clay, clay-loam, by hand texture assessment)
- Structure (single grain, massive, blocky, prismatic)
- Colour and mottling (mottled soils indicate seasonal waterlogging)
- Presence of hardpan or impervious layers
- Root distribution
- Any evidence of historic saturation
3. Percolation Test The standard percolation test measures the rate at which water moves through the soil at the depth where the absorption system will be installed.
4. Hydraulic Loading Rate Calculation From the percolation test results, the soil’s Long-Term Acceptance Rate (LTAR) is calculated, the maximum application rate of effluent in litres per square metre per day that the soil can sustainably accept without becoming saturated or allowing breakthrough.
5. Assessment Report The consultant prepares a written report including all field data, test results, calculations, and recommendations for system type and sizing. This report must be submitted with the DA to Port Macquarie Hastings Council.
The Percolation Test: What Actually Happens
Site Preparation
The consultant excavates or bores test holes at the proposed absorption area depth, typically 400-600 mm below natural surface, though some systems require deeper assessment. Multiple test holes (minimum 3, often 5-6) are excavated across the proposed land application area to capture soil variability.
Pre-Soaking
Test holes are pre-soaked with water over 12-24 hours before the test is conducted. This is a critical step that many abbreviated “quick perc tests” skip, but it is specified in AS/NZS 1547:2012 because it simulates the soil’s saturated-field condition, which is the worst-case scenario the system must handle. Pre-soaking prevents overestimating drainage rates based on initial dry-soil uptake.
Measurement Period
After pre-soaking:
- A standard depth of water (typically 150 mm) is placed in each test hole
- The depth is measured at regular intervals (every 15 minutes for 4+ hours)
- Water is refilled to 150 mm as needed to maintain the test head
- The rate of fall is recorded over the measurement period
- The stabilised (consistent) rate from the last measurement intervals is used for the calculation
The result is expressed as the time (in minutes) for the water level to fall a standard distance (e.g., 25 mm fall per minute = high percolation rate; 25 mm fall taking 90 minutes = very slow percolation).
Soil Texture Assessment
In addition to the perc test, soil texture is assessed by hand, an experienced assessor can classify soil reasonably accurately using the feel method (how the soil behaves when moist and worked between fingers). Laboratory particle size analysis can be used for more precise classification where needed.
What the Results Mean for System Type
| Percolation Rate | Soil Class | Typical System Design |
|---|---|---|
| 0-2 min/25 mm fall | Class 1 (very fast, sandy) | Cap LTAR to prevent pathogen breakthrough; AWTS or biofilter may be required before application |
| 2-10 min/25 mm fall | Class 2 (fast, sandy loam) | Good for conventional absorption trench; moderate area required |
| 10-30 min/25 mm fall | Class 3 (moderate, loam to clay-loam) | Conventional absorption suitable; area calculation yields moderate trench length |
| 30-60 min/25 mm fall | Class 4 (slow, clay-loam to clay) | Larger area required; AWTS with drip irrigation may be more practical |
| >60 min/25 mm fall | Class 5 (very slow, heavy clay) | Conventional absorption may not be feasible; specialist design required |
| No measurable percolation | Impermeable | Conventional absorption not possible; mound system, AWTS drip, or other alternative |
For the Mid North Coast specifically:
- Coastal sandy soils (Lake Cathie, Bonny Hills): Class 1-2, but often require AWTS due to water table and waterway proximity
- Basalt-derived loams (Comboyne plateau): Class 2-3, good for conventional systems
- Alluvial floodplain (Wauchope flats): Class 3-4, conventional with adequate area, but flood risk is a separate constraint
- Clay-dominated ridge soils (Wauchope hinterland): Class 4-5, often needs specialist design
Mottled Soil and Seasonal Waterlogging
During the soil profile assessment, the consultant looks carefully for mottling, grey, orange, or rust-coloured patches in the soil profile that indicate the soil is periodically saturated. Mottling is important evidence of the seasonal high water table depth.
If mottling is observed within 600 mm of the proposed absorption trench depth, the design must account for seasonal saturation. This usually means the conventional absorption trench is not suitable for those months when the water table rises, and the system must be either elevated (mound system) or the land application method changed to one that can handle saturated conditions (AWTS drip irrigation).
On the Mid North Coast, with its distinct wet seasons and La Niña variability, mottling is found at surprisingly shallow depths on many coastal and low-lying properties.
Who Conducts the Site Assessment?
The site assessment for onsite sewage management must be conducted by a suitably qualified person. In NSW, this typically means:
- A hydraulic engineer (particularly for complex or commercial sites)
- An environmental scientist or environmental consultant with specific experience in onsite wastewater assessment
- Some building designers with appropriate qualifications and experience
Port Macquarie Hastings Council will review the credentials of the person who signs the assessment report. A report prepared by someone without appropriate qualifications will not be accepted.
This is not a task for the property owner to self-assess and report. Even experienced contractors who have installed hundreds of septic systems are not the appropriate people to conduct and certify the site assessment unless they hold the relevant professional qualifications.
Cost of a Site Assessment and Percolation Test
On the Mid North Coast in 2026, a typical site assessment including percolation test and report:
| Site Type | Indicative Cost |
|---|---|
| Simple rural lot, straightforward soils | $800 to $1,500 |
| Coastal lot or constrained site | $1,200 to $2,500 |
| Complex site (flood risk, waterway proximity, multiple systems) | $2,000 to $5,000+ |
This cost is in addition to the system design, installation, and Council approval fees. See our full installation cost guide for the complete picture.
Timing the Assessment: Seasonal Considerations
In the Port Macquarie area, the percolation test gives the most realistic results when conducted in the wetter months (June, September) rather than in the dry summer period. An assessment conducted in January after months of dry weather may show excellent percolation rates that aren’t representative of the wet-season conditions when the system is actually most stressed.
Consultants experienced in Mid North Coast conditions will factor seasonal timing into their assessment and report. If there is a need to assess in dry conditions, a conservative factor is applied to the LTAR calculation.
FAQ
Do I need a percolation test for an AWTS installation? The level of soil assessment required for an AWTS with drip irrigation is sometimes less intensive than for a conventional absorption trench, the drip irrigation design is based on controlled application rate rather than passive soil percolation. However, the site still needs to be assessed for waterway setbacks, slope, potential for surface ponding, and suitability of the land application area for the proposed use. A simplified assessment may suffice, but confirm requirements with Council before commissioning.
Can I use a percolation test result from a neighbouring property? No. Soil conditions vary across short distances on the Mid North Coast, particularly in areas with volcanic, sandstone, and alluvial geology mixing. A test from the neighbouring lot is not acceptable evidence for your site’s hydraulic capacity.
What happens if my perc test shows the soil won’t absorb effluent? A failing perc test (very slow or impermeable soil) does not mean you cannot have an onsite sewage system, it means a conventional absorption trench will not work. Alternative systems (AWTS with drip irrigation, mound systems, or biofilter systems) may still be feasible. Your consultant will recommend alternatives based on the full site assessment.
Who organises the percolation test, the owner or the installer? In most cases, the property owner commissions the site assessment and perc test directly from a consultant as part of the pre-DA preparation. Some system installers have arrangements with consultants and can organise it for you. Either way, the consultant must be independent and appropriately qualified.
How long are percolation test results valid? Results are generally valid for the duration of the approval process for the specific system they were conducted for. If significant time passes (3+ years) or if site conditions change (fill added, drainage altered), Council may require an updated assessment before approving a DA.
Planning a new septic system in the Port Macquarie or Hastings region? Contact us for a referral to an experienced consultant who knows the local soil conditions, or to discuss how to read an existing assessment report for a property you are buying.