The EPA Code of Practice
The EPA's Code of Practice: Domestic Waste Water Treatment Systems (Population Equivalent ≤ 10) sets out how to assess a site and choose and design a system for a single house. Councils expect applications to follow it, with the results on the Code's site characterisation form.
What the assessment involves
- Desk study: the published maps for the site, including groundwater vulnerability, aquifer category, subsoil, soil, and nearby wells, watercourses and source protection areas. Together these set the groundwater protection response (GWPR) for the site, which decides how strict the rest of the assessment is.
- Visual assessment: slope, vegetation (rushes are a sign of wet ground), outcropping rock, drains, wells and houses nearby, and whether the separation distances below can be met.
- Trial hole: dug to see the soil and subsoil layers and how deep the water table and bedrock are.
- Percolation tests: holes filled with water and timed as it soaks away.
The trial hole
- Dug next to, but not in, the proposed percolation area, typically about 1 m by 6 m with a sloped end.
- At least 2.1 m deep, or to bedrock, over locally important or poor aquifers; at least 3 m where possible over a regionally important aquifer or where the groundwater protection response is stricter.
- Left open for at least 48 hours so any water table can show itself, fenced off and covered.
How the percolation test works (T-test and P-test)
The 2021 Code renamed the tests: the subsurface test was the T-test, and the surface test was the P-test. The method is the same for both:
- Three test holes are dug beside the proposed percolation area, each 300 mm × 300 mm and 400 mm deep. For the subsurface test the top of the hole sits at the level of the percolation pipe, so its base is roughly 950 mm below ground if the pipe is at 550 mm. The surface test is done at ground level.
- Each hole is pre-soaked twice, filling it to 400 mm.
- The holes are refilled and timed as the water drops. The percolation value is the average time, in minutes, for the water to fall 25 mm, averaged over the three holes.
The subsurface test is used when there's enough depth of unsaturated subsoil for the pipes to sit in it. Where the system will be raised or mounded, both tests are needed. Any test needs at least 0.5 m of unsaturated soil or subsoil.
What percolation value passes?
| System | Acceptable percolation value (minutes per 25 mm) |
|---|---|
| Septic tank and percolation area | 3 to 50 |
| Secondary treatment system and soil polishing filter | 3 to 75, up to 90 or 120 for some low-pressure and drip designs |
| Tertiary treatment system and infiltration area | 3 to 75 |
- Under 3: the water drains too fast (gravel, sand, fissured rock) to treat the effluent. Importing slower soil may help.
- Over 120: the ground is too slow (usually clay), and the site can't discharge to ground at all. Discharging to a stream instead needs a licence from the council.
- As a rough guide, the Code's own test data puts gravel at 2 to 8, sand 5 to 18, silt 11 to 31, silt/clay 18 to 43 and clay above 41.
How deep does the ground need to be?
There must be enough unsaturated soil or subsoil between the bottom of the percolation area and the water table or bedrock. For a septic tank with percolation trenches, that's 1.2 m in the least sensitive areas and 2 m in most others. Over the most sensitive groundwater a septic tank isn't acceptable at all. A secondary treatment system needs less: 0.9 m to 1.8 m. Where the ground is shallower but at least 0.5 m deep, a raised percolation area may be possible.
How big is a percolation area?
For a septic tank, the Code sets a minimum total trench length of 18 m per person, in trenches 500 mm wide and no more than 18 m long each:
| People in the house | 4 | 5 | 6 | 7 | 8 | 10 |
|---|---|---|---|---|---|---|
| Minimum trench length | 72 m | 90 m | 108 m | 126 m | 144 m | 180 m |
So a house designed for six people needs at least six 18 m trenches, laid parallel to the contour on a sloping site. Secondary and tertiary systems usually need a smaller area.
Septic tank and percolation area distances
The Code's Table 6.2 sets minimum distances from the whole system, both the tank and the percolation area. If any can't be met, the site isn't suitable.
| From | Minimum distance |
|---|---|
| Your own house | 7 m from the tank; 10 m from the percolation area |
| A neighbour's house | 7 m from the tank; 10 m from the percolation area |
| Site boundary | 3 m |
| Road | 4 m |
| Trees | 3 m (from the canopy spread) |
| Stream, watercourse, open drain or ditch | 10 m |
| Lake or foreshore | 50 m |
| Surface water soakaway | 5 m |
| Another treatment system | 10 m |
| Karst feature (swallow hole, cave) | 15 m |
| Domestic well, up-gradient of the system | 15 m |
| Domestic well, alongside | 25 m |
| Domestic well, down-gradient | 30 m to 60 m, depending on the subsoil |
| Public or group water scheme well | 60 m |
The ground under the system must also be no steeper than 1 in 8. Distances from monuments and protected nature sites are set case by case by the council.
What the results mean
- Water that drains too slowly (heavy clay, wet ground) won't take the effluent away.
- Water that drains too fast (sand, gravel, fissured rock) doesn't treat it before it reaches groundwater.
- A high water table or shallow rock leaves too little unsaturated ground for treatment.
Depending on the results, the system might be a septic tank with a percolation area, or a packaged secondary treatment system with a polishing filter, sometimes raised above the natural ground. Some sites can't take any system.
Common questions
Can I have a septic tank without a percolation area?
No. A septic tank only settles solids; the Code says it doesn't fully treat wastewater. The percolation area, or a polishing filter after a treatment system, is where the treatment happens.
How far must a septic tank be from the house?
At least 7 m for the tank and 10 m for the percolation area, from your own house and from neighbours'.
Who can do the percolation test?
Councils expect a trained site assessor working to the EPA Code. Many councils keep a list, and the result goes on the Code's site characterisation form with the planning application.
Hints from public data, before you dig
- Inside a sewered area? A connection may avoid the question.
- Poorly drained (gley) soils, peat, or a shallow water strike in nearby wells suggest a difficult site.
- Extreme groundwater vulnerability or a regionally important aquifer means a stricter response.
After it's built
Every domestic wastewater treatment system must be registered with the local authority, and the EPA's National Inspection Plan targets inspections at higher-risk areas.
Sources
- EPA, Code of Practice: Domestic Waste Water Treatment Systems (Population Equivalent ≤ 10) (2021): Tables 6.2 to 6.4 and 7.2, section 5.4 and Appendix D
- EPA, National Inspection Plan for domestic waste water treatment systems
Last reviewed 2026-10-07.