Where these numbers come from
Not from a search result, and not from anyone's experience. The percentages on this page were computed by geometric intersection of NSW Government soil landscape polygons with NSW Spatial Services suburb boundaries, so they are area-weighted shares of the actual suburb outline rather than estimates. Every phrase in quotation marks is verbatim from the official soil landscape unit report for that unit, published by the NSW Department of Climate Change, Energy, the Environment and Water via eSPADE.
Two map sheets meet on this coast, which matters for precision. Woy Woy, Umina Beach and Ettalong Beach straddle the boundary between the Sydney 1:100,000 sheet (Chapman and Murphy, 1989) and the 1993 Gosford sheet (Murphy). The same unit name therefore appears twice with two slightly different report texts, and we cite which sheet a quote comes from rather than merging them.
The paradox, unpacked
Permeability and watertable depth are two different properties, and people conflate them constantly. Permeability describes how fast water travels through the material. A watertable is the depth below which the ground is already saturated. Sand can be excellent at the first while being poor at the second, and on the peninsula it is exactly that.
The Woy Woy unit is described on the 1993 Gosford sheet as "deep (>200 cm) Siliceous Sands (Uc1.22, Uc5.11) and occasional Podzols (Uc2.3) on well-drained older beach ridges, Humus Podzols (Uc4.22, Uc4.24, Uc2.21) in poorly drained areas". Note that both conditions are in the same sentence: well drained here, poorly drained a short distance away. Its stated limitations include "permanently high watertables, localised flooding, periodic waterlogging in depressions". The Sydney sheet version adds the landscape line "Watertable at a depth of <200 cm" and lists "Non-cohesive soil" among the limitations.
So what does that do to a lawn? Rain that lands on the surface disappears almost instantly, which is why peninsula gardeners often describe their soil as impossible to keep wet. But after a sustained wet spell, the saturated zone rises toward the surface, and the ground that was dry a week ago is now wet from below rather than from above. A natural lawn on that ground swings between drought stress and waterlogging without ever behaving like the loam it is being fertilised as.
For a synthetic turf base the implication is precise, and it is the opposite of what most installers reach for. Drainage capacity is not the constraint here. There is no need to build a deep soakaway into ground that already drains faster than anything you could construct. The constraints are two others: keeping a non-cohesive material stable under load, and not excavating so deep that the base sits in a zone that saturates seasonally.
Stability in non-cohesive sand
Non-cohesive means the particles do not stick to each other. The Woy Woy report is direct about the consequence: the foundation hazard is rated low to moderate, but the "poorly sorted unconsolidated sands may require compaction, and shoring of foundations may be required", with limitations including "soils with low wet bearing strength and localised organic soils" and "fluctuating high watertables".
Practically, that is a warning about settlement rather than about water. Loose sand under a lightly compacted base will keep finding a denser arrangement under foot traffic, and the lawn that was flat at handover develops soft hollows along the paths people actually walk. The defence is unremarkable but non-negotiable: proper compaction of the sand before any base material goes on, a structural layer of road base that is wetted and plate compacted rather than simply raked out, at least two compaction passes, and edge restraint that actually restrains, because non-cohesive material migrates sideways at an unconfined edge.
None of that costs much more than doing it badly. It just takes longer, which is precisely why it is the first thing a cheap quote leaves out. We walk through the whole sequence in the laying guide.
Umina Beach: a quarter of the suburb is fill
The strongest base-preparation warning on this coast is not about sand at all. Umina Beach maps, at 100 per cent coverage, as 47.4 per cent Woy Woy unit, 24.7 per cent Disturbed Terrain, 15.7 per cent Watagan, 5.4 per cent Hawkesbury and 3.3 per cent Tuggerah, on the Sydney sheet.
Disturbed Terrain is artificial fill. The Sydney sheet describes it as "turfed fill areas commonly capped with up to 40 cm of sandy loam or up to 60 cm of compacted clay over fill or waste materials", with geology recorded simply as "Artificial fill". Its limitations are "dependent on nature of fill material. Mass movement hazard, unconsolidated low wet strength materials, impermeable soil, poor drainage, localised very low fertility and toxic materials". Surface movement potential is "Highly variable, depending on materials and degree of compaction". The 1993 Gosford sheet version of the unit adds, for urban land, that "geotechnical engineering advice should be sought prior to approval for development being given".
That is 24.7 per cent of a suburb where the honest answer to "what is under this lawn?" is that nobody knows until it is opened up. And note the sting in the tail: on fill capped with compacted clay, you can be standing in a famously sandy suburb on top of ground the report describes as "impermeable soil, poor drainage". The sand assumption fails completely, and it fails without warning from the street.
What that should change about how you buy. Do not accept a fixed price quoted from a photograph on a Umina block. Ask the installer what they expect to find, what they will do if it is fill, and how the price moves if it is. A quote that addresses that question is from someone who has worked the suburb. A quote that does not is a number that will change later, in a conversation you will enjoy less. See our Umina Beach page and the mechanics in the cost guide.
Umina is not alone, only extreme. East Gosford carries 6.5 per cent Disturbed Terrain at 98.3 per cent coverage, and Woy Woy 1.5 per cent on its mapped portion. The same questions apply, proportionately.
Ettalong Beach and Woy Woy, with the caveats attached
Ettalong Beach is only 83.7 per cent mapped, so every figure describes that mapped portion. Of it, 68.9 per cent is the Woy Woy unit, 8.2 per cent Tuggerah and 6.2 per cent Narrabeen. That is the cleanest expression of the paradox anywhere on the peninsula: more than two thirds of mapped Ettalong is deep siliceous sand carrying a watertable under two metres. Free drainage is not the problem there. A high watertable under a compacted base is.
Woy Woy is the awkward one, and its 86.7 per cent coverage is the least of it. Across the mapped portion there are seventeen units drawn from two map sheets, and not one of them exceeds 17 per cent: Hawkesbury 16.5 per cent, Lambert 14.5 per cent, Tuggerah 14.3 per cent, a second Lambert mapping at 10.8 per cent, then a long tail. Those units behave nothing like each other. Hawkesbury is "shallow, stony, highly permeable soil" over sandstone. Lambert has "total soil depth is <50 cm" with "seasonally perched watertables". Tuggerah is deep dune sand, "non-cohesive, highly permeable soils" with "localised flooding and permanently high watertables".
The conclusion for Woy Woy is not a hedge, it is the finding: this is the most heterogeneous suburb in the corridor and a site inspection is the actual answer rather than a legal disclaimer at the bottom of a quote. Shallow rock and deep sand genuinely sit within a few streets of each other. More on the suburb at our Woy Woy page.
Green Point we will barely quantify at all, because only 58.6 per cent of it carries mapping. On that mapped portion the two largest units are Erina at 23.9 per cent and Watagan at 21.1 per cent, which reads as clay hillside rather than sand. The rest is unmapped and we will not guess at it.
Over the hill, the problem inverts
Cross from the peninsula to the suburbs behind Gosford and everything said above stops applying. The Erina unit, which dominates Erina (57.7 per cent at 98.4 per cent coverage), Avoca Beach (76.5 per cent of a 90.3 per cent mapped polygon), Kincumber (65.9 per cent, 93.0 per cent coverage), East Gosford (64.4 per cent), Copacabana (72.9 per cent, 96.4 per cent coverage) and Wyoming (56.8 per cent), is rated by its report as carrying "a moderate to high foundation hazard", with localised limitations including "soils with moderate shrink-swell, high plasticity and low wet bearing strength" and landscape limitations including "seasonal waterlogging of footslopes" and subsoil "Low permeability".
Add the steeper Watagan unit, which is 32.2 per cent of Wyoming, 20.2 per cent of Copacabana and 19.2 per cent of East Gosford, and which the report rates "High foundation hazard" with "steep slopes", "highly plastic subsoil, low wet bearing strength, moderate shrink-swell" and total soil depth varying "from <50-200 cm".
Here the base genuinely is a drainage structure. Water is not going down through low-permeability subsoil at any useful rate, so it has to be moved sideways on a deliberate fall to a point where it can leave. And because the subsoil moves with its moisture content, the base has to be thick enough and compacted enough that seasonal movement below does not telegraph through to the surface.
Two suburbs, one council, forty minutes apart, and opposite base designs. That is the reason a Central Coast quote should never be a per-square-metre rate applied from a spreadsheet, and it is why what the installer does on site matters more here than the brand on the roll.
What we cannot tell you
Being useful means being clear about the edges of the data. We cannot tell you an AS 2870 site class for your suburb, because no free primary source assigns one and inventing it would be worse than useless. We cannot tell you an excavation depth as a function of soil type, because no Australian Standard or government source specifies one for artificial turf. We cannot describe the unmapped 41.4 per cent of Green Point, the 16.3 per cent of Ettalong Beach or the 13.3 per cent of Woy Woy, because no unit is assigned there. And we cannot tell you what is under your particular yard, because 1:100,000 mapping does not work at that scale.
What the data does do is tell you which questions to ask, and let you recognise an installer who already knows the answers.
Soil data: Murphy C.L., 1993, Soil Landscapes of the Gosford-Lake Macquarie 1:100,000 Sheets, NSW Department of Conservation and Land Management; Chapman G.A. and Murphy C.L., 1989, Soil Landscapes of the Sydney 1:100,000 Sheet, Soil Conservation Service of NSW; dataset Soil Landscapes of Central and Eastern NSW v3.0.1, DCCEEW 2024. Licensed Creative Commons Attribution.
Questions about the ground under your lawn
How can sand be free-draining and have a high watertable at the same time?
They describe different things. Permeability is how fast water moves through the material; a watertable is the level below which the ground is already saturated. The Woy Woy soil landscape is both: the NSW reports describe deep siliceous sands with high permeability, and the same reports list "permanently high watertables" with the Sydney sheet recording the watertable "at a depth of <200 cm". Water leaves the surface quickly and then meets ground that is already full.
Does that mean I cannot put turf on the peninsula?
No. It means the base is designed for a different problem than most installers assume. You are not solving drainage capacity, because the sand has plenty. You are solving stability in non-cohesive material and making sure the excavation does not go looking for groundwater. That is a design decision, not an obstacle.
What is Disturbed Terrain and why does Umina Beach have so much of it?
It is the soil landscape mapping term for artificial fill: ground that was placed there rather than formed there. Umina Beach maps at 24.7 per cent Disturbed Terrain, the highest share of any suburb in the NSW corridor we cover. Why there is that much is not something the mapping explains and we are not going to speculate. What the unit report does say is what it means: "unconsolidated low wet bearing strength materials, impermeable soils, poor drainage", with surface movement "highly variable, depending on materials and degree of compaction".
Are these percentages about my actual backyard?
No, and this is important. They are area-weighted shares of the whole suburb polygon, including roads, parks and water, computed at a mapping scale of 1:100,000. They tell you what is likely and what to ask about. They cannot tell you what is under one particular yard, and the reports describe themselves as an inventory of soil and landscape properties rather than a substitute for site investigation.
Why do you keep quoting a coverage percentage?
Because in three of these suburbs a meaningful share of the suburb polygon carries no soil landscape mapping at all, and quoting a percentage without saying so would overstate what we know. Woy Woy is 86.7 per cent mapped, Ettalong Beach 83.7 per cent and Green Point only 58.6 per cent. Every figure we publish for those suburbs describes the mapped portion. The unmapped remainder is genuinely unknown and we will not infer it from the neighbours.
Does AS 2870 tell my installer how deep to dig?
No. AS 2870 is a residential footings and slabs standard called up by the National Construction Code for houses, garages and sheds. It has nothing to say about landscaping and imposes nothing on a turf installer. There is also no Australian Standard or government source that specifies excavation depth or base preparation for artificial turf as a function of soil type. Anyone citing one is inventing it.
Should I get a geotechnical report before laying turf?
For a domestic lawn, almost never. It is worth knowing, though, that for urban development on Disturbed Terrain the unit report itself says "geotechnical engineering advice should be sought prior to approval for development being given". That is about buildings, not lawns. The proportionate version for a turf job is simply this: on ground that might be fill, do not accept a fixed price given over the phone, and make sure the quote states what happens to the number if the excavation finds something unexpected.
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