The absorption trench: how it's built, how long it must be, and the reserve area nobody mentions
- Design loading rate runs from 15 mm/day on massive sandy loam to 3 mm/day on weakly structured light clay — and heavy clay takes no primary effluent at all.
- AS/NZS 1547 requires a 100% reserve area: a second land application area, held empty.
- Setbacks disagree by jurisdiction. A bore is 20–50 m in Victoria, 100 m for WaterNSW, 250 m at Camden.
- A trench is expected to last about 25 years.
AS/NZS 1547:2012 fixes the geometry most quotes leave out. The base sits typically 400 mm below the surface, though WaterNSW notes trenches of 500 to 700 mm. Width is typically 300 to 450 mm, minimum 200 mm, maximum 600 mm, with WaterNSW allowing up to 1,000 mm. The aggregate layer is 250 mm of 20 to 40 mm stone around a 100 mm slotted distribution pipe, covered by geotextile so soil cannot migrate in, then about 150 mm of native soil, mounded roughly 50 mm above grade so rain runs off. Queensland caps the design loading rate for primary effluent at 25 to 30 mm a day even in the best soils. The setbacks disagree by authority: a bore or well is 50 m in Victoria on category 1 and 2a soils and 20 m on 2b to 6, 250 m under Camden Council, 100 m under WaterNSW, 50 m for Douglas Shire while a Queensland commercial guide says 30 m; a dam is 300 m potable and 60 m non-potable in Victoria, 40 m for Camden and WaterNSW, 50 m in Douglas Shire; the boundary is 6 m up-slope and 3 m down-slope in Victoria and WaterNSW, 12 m and 6 m for Camden. Surf Coast Shire puts the performance life at about 25 years, and a 100% reserve area is required by AS/NZS 1547 and the Victoria Government Gazette even though one guideline says 50%.
Length comes from the AS/NZS 1547 equation: daily flow divided by the design loading rate times the trench width. A 3-bedroom house at 6 people and 180 litres a person needs 1,080 litres a day; on a category 2 soil with a rate of 15 mm a day and a 0.6 m wide trench that is 120 metres of drain. Queensland's cap of 25 to 30 mm a day for primary effluent, even in sand, keeps trench length long by design. The reserve area has to be the same size, on the same block, with the same setbacks: a reserve inside the 20 to 250 m bore setback is not a reserve.
The trench is where your sewage is actually treated, and it is the part of the system nobody photographs. It has no brand, no warranty and no salesperson. It is a slot in the ground, filled with stone, and its dimensions are dictated by a soil report you probably have not read.
Get it wrong and there is no repair in the ordinary sense: you dig a new one, in ground you may not have.
What is actually in the ground
- Topsoil, 150 mm, backfilled to the original level and mounded about 50 mm proud so the trench sheds rain and can settle.
- Geotextile filter cloth, laid over the aggregate so soil cannot wash down and blind the stone.
- Aggregate, 250 mm deep, of 20–40 mm stone, with a 100 mm slotted PVC pipe distributing effluent along the run.
- Depth to the base: about 400 mm below the surface in Victorian guidance; WaterNSW describes trenches at 500–700 mm.
- Width, 300–450 mm typically, 200 mm minimum and 600 mm maximum — WaterNSW allows up to 1,000 mm for arch trenches.
None of that is decorative. The geotextile is the difference between a trench and a soakage pit that silts up. The mound is why the trench is not a bathtub after the first storm. And the pipe is only a distributor — it does not treat anything, and it stops being useful the moment one end of the run does all the work.
The number your entire quote hangs on
Design loading rate is how many litres a square metre of trench floor absorbs each day, expressed as millimetres per day. It is set by soil category, and AS/NZS 1547 publishes the table that decides whether your block can have a trench at all.
| Category | Soil | DLR, primary effluent | DLR, secondary |
|---|---|---|---|
| 1 | Gravels and sands, structureless | not permitted¹ | 25 mm/day |
| 2a | Sandy loams, weakly structured | not permitted¹ | 24 |
| 2b | Sandy loams, massive | 15 | 30 |
| 3a | Loams, highly or moderately structured | 15 | 30 |
| 3b | Loams, weakly structured or massive | 10 | 30 |
| 4a/b | Clay loams, strongly or moderately structured | 8 | 20 |
| 4c | Clay loams, massive | 5 | 10 |
| 5a | Light clays, strongly structured | 5 | 12 |
| 5b | Light clays, moderately structured | 3 | 10 |
| 5c | Light clays, weakly structured or massive | not recommended | 8 |
| 6a/b/c | Medium to heavy clays | not recommended | 5² |
Two things fall out of that table immediately. First, the range is five-fold: a massive sandy loam takes 15 mm a day, a moderately structured light clay takes 3. The same four-person household needs roughly 67 metres of 600 mm trench on the first soil and around 333 metres on the second. Second, on heavy clay a conventional trench is simply not recommended — and the secondary column shows the way out. Treat the effluent to secondary standard and a medium clay will accept 5 mm/day where it accepted nothing before. That is what an aerated system buys on a bad block: not cleaner water for its own sake, but permission to use the soil you have.
The reserve area, which is not optional
Almost nobody is told this before they buy the block. AS/NZS 1547, section 4.2.3.4, requires a 100% reserve area for primary effluent absorption systems: a second land application area, the same size as the first, kept in reserve for when the first fails or needs resting.
Victoria makes it mandatory in every Special Water Supply Catchment, and requires it for trench and bed systems generally unless a council is satisfied by a land capability assessment that the risk is low. Camden Council requires it for trenches, beds and mounds. Clarence Valley requires it. So do Douglas Shire in Queensland and Sorell in Tasmania.
It must be land separate from the working trench, excluded from traffic, and left free of anything that would stop a trench being dug there in fifteen years’ time. Which means the real footprint of a conventional system is double the number in your quote — and the shed, the driveway or the second dwelling you were planning cannot go on top of it.
Setbacks, and why nobody agrees
Every jurisdiction sets its own distances, and they differ by a factor of twelve. These are the published numbers, per authority.
| From | Victoria | Camden Council (NSW) | WaterNSW | Douglas Shire (QLD) |
|---|---|---|---|---|
| Bore or well | 50 m (cat 1/2a soils) · 20 m (cat 2b–6) | 250 m | 100 m | 50 m |
| Dam | 300 m potable · 60 m non-potable | 40 m (farm dam) | 40 m | 50 m |
| Waterway | 100 m potable · 60 m non-potable | 100 m (permanent) | 100 m | 50 m |
| Property boundary | 6 m up-slope · 3 m down | 12 m up · 6 m down | 6 m up · 3 m down | — |
| Building | 6 m up-slope · 3 m down | 6 m up · 3 m down | 6 m up · 2 m down | — |
| Swimming pool | 6 m | 6 m up · 3 m down | 15 m, and down-slope | 6 m |
| Stormwater drain | 50 m open · 6 m closed | 40 m | 40 m | not stated |
Read the bore row twice. Victoria will let a trench sit 20 metres from a bore on clay soils; Camden wants 250. Neither is wrong — Victoria’s short distance applies where the soil itself is a filter, Camden’s long one where the council has decided it will not take the risk. What it means for you is that a design copied from the internet, or from the neighbouring shire, will fail approval.
One more from WaterNSW, easy to miss: a primary effluent trench must sit 15 metres from an in-ground pool or water tank and be down-slope of it. Distance alone does not satisfy the rule.
Twenty-five years, if nothing drives over it
Surf Coast Shire puts the performance life of an absorption trench at about 25 years. Very few reach it, and the reasons are almost never what owners expect.
Tree roots are the leading cause: a 25-year Gippsland study found root infiltration behind 75% of trench failures. After that comes compaction from vehicles or stock, then smearing — an excavator glazing the trench walls during construction, sealing the soil before a drop of effluent arrives — and finally simple hydraulic overloading, more litres a day than the soil category can take.
They do eventually fail though, but you can just install another and a switcher and jump between multiple trenches to give each a break to dry out every few months or so
r/AusRenovation, septic ownerThat is precisely what the reserve area is for, and Victorian guidance contemplates it: where a trench fails, a new distribution box can be installed so the soil can be rested and the flow diverted. It is a design decision, not a repair. Owners who never reserved the land have no such option.
Base typically 400 mm below the surface; WaterNSW cites 500 to 700 mm.
300 to 450 mm typical, 200 mm minimum, 600 mm maximum, up to 1,000 mm under WaterNSW.
250 mm of 20 to 40 mm stone around a 100 mm slotted pipe, under geotextile.
20 to 50 m in Victoria, 100 m under WaterNSW, 250 m under Camden Council, 50 m in Douglas Shire.
About 25 years according to Surf Coast Shire.
100% under AS/NZS 1547 and the Victoria Government Gazette; one guideline says 50%.
Flow ÷ (design loading rate × width): 1,080 L/day ÷ (15 × 0.6) = 120 m.
25 to 30 mm a day for primary effluent.
Resting a trench, and the argument the sources are having
When a trench stops accepting, two documents disagree, and it is worth knowing which one is describing your trench.
The Victoria Government Gazette contemplates rehabilitation: installing a new distribution box so the soil can be rested between doses may resolve land degradation. The Code of Practice and the Gazette both mandate the duplicate reserve area precisely so the original land application area can be rested if it fails, becomes inadequate, or needs to recover.
A commercial installer takes the opposite view — that once trenches are saturated a repair “rarely makes sense” and replacement is better.
Both are right about different trenches. A trench sealed by sodium from laundry powder has a chemistry problem, and gypsum plus rest can reverse it. A trench whose biomat has thickened for twenty-five years has a biology problem that rest does not fix, because the clogging layer is not going to un-grow.
The test that separates them is free and takes a fortnight: cut water use hard, keep the rain off with a small bund, and see whether the ponding drains. If it does, the trench was overloaded. If it does not, the trench is spent, and the reserve area you were told to keep clear is about to earn its keep.
Keep one sentence in mind while reading any quote. The soil report is the cheapest document in the project and the only one that changes the length of the trench, the size of the reserve, and therefore the price of everything that follows. It is also the document that installers are happiest to skip, because it is the one that can lose them the job.
What it costs, and what nobody publishes
No government source we could find puts a dollar figure on rejuvenating or replacing a trench. Commercial guides quote $2,000–7,000+ to build a land application area, and $3,000–8,000 to replace an absorption trench. Treat both as market prices, because that is what they are.
What is worth budgeting is the assessment that sizes it: a land capability assessment is $1,400–3,000, and it produces the soil category that determines every number on this page. Skip it and the trench in your quote is a guess.
Size the system with the tank size calculator and check whether your site supports a trench at all with the which system calculator. How a septic tank works explains why the tank hands 65–75% of the job to this trench, what a septic system costs breaks the quote into lines, and septic safe products covers the sodium that closes the soil you just paid to open.
The reserve area is the most valuable thing on this page and the least discussed. Everyone negotiates the tank, argues about the trench length, and then builds a shed on the only piece of ground the system can retreat to. Fifteen years later the trench seals — as trenches do — and the choice is an aerated system at $15,000–25,000 or a legal problem, because there is nowhere left to dig. When you look at a rural block, walk the reserve area before you walk the house site. It is the cheapest insurance in on-site wastewater, and it costs nothing but restraint.
The reserve area is worth defending because the alternative to it is a new system. Size the tank first in what size septic tank, protect the soil with septic-safe products, and if your block has no room for a trench at all, look at a worm farm system or a composting toilet before you look at anything else.
Frequently asked questions
How deep and wide is an absorption trench?
Typically 300–450 mm wide, with a minimum of 200 mm and a maximum of 600 mm; WaterNSW allows up to 1,000 mm for arch trenches. The base sits about 400 mm below the surface in Victorian guidance, and WaterNSW puts trenches at 500–700 mm deep. Inside: 250 mm of 20–40 mm aggregate, a 100 mm slotted pipe, geotextile over the top, then 150 mm of native soil mounded about 50 mm proud of the surface.
What is a design loading rate?
The litres of effluent each square metre of trench floor can absorb per day, expressed in millimetres per day. AS/NZS 1547 assigns it by soil category: 15 mm/day for massive sandy loams and highly structured loams, 8 for structured clay loams, 5 for strongly structured light clays, 3 for moderately structured light clays. Weakly structured light clays and medium to heavy clays are not recommended for primary effluent at all.
Do I really need a reserve area?
Yes. AS/NZS 1547 section 4.2.3.4 requires a 100% reserve for primary effluent absorption systems — a second area the same size as the first, kept free of traffic and structures. Victoria mandates it in all Special Water Supply Catchments; Camden, Clarence Valley, Douglas Shire and Sorell all require it.
How long does a trench last?
Surf Coast Shire puts the performance life at about 25 years. Tree roots are the leading cause of early failure — 75% of cases in a 25-year Gippsland study — followed by compaction, wall smearing during installation, and overloading.
Researcher & editor, on-site wastewater
Researches and edits independent guides on septic systems and AWTS across Australia, cross-checking AS/NZS 1547, council requirements, real prices and owner experiences.