Physical geography

The Sydney Basin bioregion extends along the eastern Australian seaboard from the Hawkesbury River estuary in the north to just south of Durras on the southern NSW coast. Its western boundary is defined by the geological Sydney Basin and its eastern boundary by the Australian coastline (Figure 8). Most of the western boundary is east of the Great Divide with the headwaters of the Wollondilly and Shoalhaven rivers lying to the west of the bioregion. In the north-west, the bioregion crosses the Great Dividing Range into the westerly draining Macquarie river basin. The bioregion contains the major cities of Sydney and Wollongong, as well as some smaller urban centres including Lithgow in the west, Nowra in the south and a number of towns in the Southern Highlands. Just over 40% of the bioregion is used for conservation, including the Blue Mountains National Park, Wollemi National Park and Morton National Park.

The Sydney Basin bioregion is defined by geology in the west, the Hunter Range topographic divide in the north and the coastline in the east. Land surface elevations range from sea level to 1260 mAHD (metres Australian Height Datum) at Mount Coricudgy in the north (Figure 8). Much of the bioregion is characterised by relatively rugged terrain associated with the heavily dissected rock outcrops of the Hawkesbury Sandstone. Inland from Sydney and Nowra and south of Wollongong, the terrain is flatter, reflecting the development of coastal plains from the interactions of rivers with the sea. The tableland areas around Berrima in the Southern Highlands and west of Mount Coricudgy also have flatter relief. Figure 9 shows some very marked breaks of slope where the rugged Hawkesbury Sandstone country meets the coastal plains, with dramatic cliffs, giving way to short, steep colluvial slopes and then flat alluvial environments.

Two major river basins drain the bioregion (Figure 10). They are the Hawkesbury-Nepean river basin, which covers 16,049 km2 or 65% of the bioregion and the Shoalhaven river basin (2905 km2; 12% of bioregion) in the south. The remaining area includes three small coastal river basins (4805 km2) – Sydney Coast-Georges River; Wollongong Coast and part of Clyde River-Jervis Bay – and a small part of the Macquarie-Bogan river basin (1530 km2), which lies to the west of the Sydney Basin bioregion. Major tributaries within the Hawkesbury-Nepean river basin include the Macdonald, Colo and Wollondilly rivers. Much of the river basin upstream of Lake Burragorang, the water supply reservoir created by Warragamba Dam, lies outside the bioregion. Similarly, the headwaters of the Shoalhaven River are outside the bioregion. The Cudgegong River, which flows westward into the Macquarie River, rises in the Sydney Basin bioregion. Section 1.1 .5 provides more detail about the surface water hydrology of the bioregion.

The coast is characterised by drowned river valleys and coastal lake systems. Around Sydney, these include Broken Bay, Port Jackson, Botany Bay and Port Hacking; moving southward, they include Lake Illawarra, Sussex Inlet, Lake Conjola and Burrill Lake – all are important water bodies for recreation and habitat (Figure 10).

Figure 8

Figure 8 Surface elevation of the Sydney Basin bioregion

Data: Bioregional Assessment Programme (Dataset 1, Dataset 2); Geoscience Australia (Dataset 3)

Figure 9

Figure 9 Land surface slopes of the Sydney Basin bioregion

Data: CSIRO (Dataset 4)

Figure 10

Figure 10 Major river basins and selected surface water bodies of the Sydney Basin bioregion

Data: Bureau of Meteorology (Dataset 5); Bioregional Assessment Programme (Dataset 6) Physiography and soils

The Sydney Basin bioregion is comprised of four physiographic regions of the Australian Soil Resource Information System (ASRIS) (Figure 11). Physiographic regions are defined by an internal coherence of their landform characteristics and underlying geology (Jennings and Mabbutt, 1986). They are considered to be areas of similar landform evolutionary history, which have given rise to similar groups of regolith materials, such that the resultant mapped units can be described in terms of landform, underlying geology, regolith and soils (Pain et al., 2011). The main ASRIS classes are (i) Hawkesbury-Shoalhaven Plateaus, (ii) Bathurst Tablelands, (iii) Cumberland Lowland and (iv) Monaro Fall. Figure 11 shows a tiny intersect with the Goulburn Corridor in the north, which likely reflects scale of mapping. It is not included in Table 6. The Interim Biogeographic Regionalisation for Australia (IBRA) subregions shown in Figure 6 of Section 1.1 .1 broadly conform to these physiographic regions (Table 6). Table 7 summarises the IBRA subregions by geology, landform and soils.

Table 6 Australian Soil Resource Information System physiographic classes in the Sydney Basin bioregion and corresponding Interim Biogeographic Regionalisation for Australia subregions


ASRISa class name

Area in bioregion (km2) (%)

Corresponding IBRAb subregions


Hawkesbury-Shoalhaven Plateaus



Wollemi, Yengo, Pittwater, Burragorang, Sydney Cataract, Moss Vale, Illawarra, Ettrema


Bathurst Tablelands



Capertee Valley, Capertee Uplands


Cumberland Lowland





Monaro Fall




aAustralian Soil Resource Information System

bInterim Biogeographic Regionalisation for Australia

Data: CSIRO (Dataset 7)

Figure 11

Figure 11 Australian Soil Resource Information System (ASRIS) physiographic classes and codes in the Sydney Basin bioregion

Data: CSIRO (Dataset 7)

Table 7 Geology, landforms and typical soils of the main Interim Biogeographic Regionalisation for Australia subregions in the Sydney Basin subregion

IBRAa subregion


Characteristic landforms

Typical soils


Hawkesbury Sandstone and equivalent quartz sandstones of Narrabeen Group, subhorizontal bedding, strong vertical joint patterns. A few volcanic necks.

Highest part of the Blue Mountains. Sandstone plateau with benched rock outcrops. Creek directions controlled by jointing deep gorge of the Capertee and Wolgan rivers.

Thin sands or deep yellow earths on plateau, thin texture contrast soils on shale benches. Organic sands in swamps and joint crevices, bouldery slope debris below cliffs, sandy alluvium in pockets along the streams. Red brown structured loams on basalts.


Hawkesbury Sandstone, valleys incised to Narrabeen Group sandstone, a few volcanic necks and basalt caps, Quaternary sandy alluvium and high level sands on Mellong Range and Maroota. Quaternary muddy sands in Hawkesbury upper estuary.

Benched sandstone plateau with steep slopes into narrow valleys with low cliff lines on Narrabeen sandstone. Structurally controlled subrectangular drainage pattern. Northern end of Lapstone monocline controls Mellong Range. Hawkesbury River gorge cuts across the subregion, tributary streams dammed by levees form freshwater swamps adjacent to the river.

Shallow quartz sands on plateau, some areas of deep yellow earth and patches of Podosol development on sandstone benches and in all Cenozoic and Quaternary high-level sands. Texture contrast soils on shales, deep clean sands in alluvium. Red brown structured loams and clay loams on basalt.


Hawkesbury Sandstone with thin ridge cappings of Ashfield Shale. Narrabeen Group sandstones exposed in valleys and along the coast. Quaternary coastal sands.

Hornsby plateau of quartz sandstone with occasional shale caps. Small beach, dune and lagoon barrier systems. Steep coastal cliffs and rock platforms.

Deep yellow earths or rocky outcrop on plateau tops. Uniform and texture contrast soils on sandstones and shale slopes. Loamy sands in alluvium along creeks, clean quartz sands with moderate shell content on beaches and frontal dunes. Organic sands and muds in estuaries.


Permian and Triassic sandstones and shales on the western edge of the Basin. Limited basalt caps.

Rolling hills on a sandstone plateau with deep gorges and sandstone cliffs in Burragorang valley.

Rocky outcrops, texture contrast soils and uniform sands on sandstone. Bouldery debris with sandy clay matrix below cliffs. Rich loams in alluvium.


Hawkesbury Sandstone on the coastal edge of the Basin above the Illawarra escarpment. Quaternary sands and muds in Georges River and Botany Bay.

Sandstone plateau with shallow creeks flowing through hanging swamps in the highest parts ramping down to low hills in the Georges River and Botany Bay. Coastal cliffs north of the Illawarra. Large barrier system with beach, dunes, swamps, and estuary at Kurnell.

Deep sands and clayey sands with peat in hanging swamps, yellow earths on better drained sandstone ridges. Siliceous sands in younger dunes and well-developed podzols in older dunes. Organic sands in swamps and estuary.

Moss Vale

Triassic Wianamatta Group shales, Paleogene and Neogene basalts and trachyte intrusions, large Quaternary peat swamp.

Shale and basalt plateau with rolling hills and shallow valleys. Very large peat swamp at Wingecarribee.

Structured red and red brown clay loams and loams, and loamy alluvium with high fertility. Areas of sandstone at the margins thin, waterlogged sandy soils. Organic peat in swamps. Stony slope debris on larger intrusions.


Permian siltstones, shale, sandstones and interbedded volcanics on and below the coastal escarpment. Quaternary alluvium and coastal sands.

Vegetated cliff faces on coastal escarpment with waterfalls and steep streams. Bouldery debris slopes with sandy clay matrix and low hills and alluvial valleys on coastal ramp. Barrier systems at Lake Illawarra and Nowra.

Structured red and red brown loams and clay loams with some areas of mellow texture contrast soils. Fertility high and good water holding capacity. Siliceous sands on beaches and dunes, podzol profiles in older dunes, peaty sands and organic silts in swamps and estuaries.


Permian horizontal quartz sandstone alternating with shales. Deep gorges expose Silurian volcanics and Carboniferous granite in underlying Lachlan Fold Belt. Limited Paleogene and Neogene basalt with river gravels.

Low stepped hills on plateau with deeply incised streams off plateau edge below waterfalls on the escarpment.

Alternating sandstone and shale create bare rock benches and soil benches with shallow, often saturated sand. Structured red brown clay loams on basalt.


Permian Shoalhaven Group conglomerates, sandstones, and shales with coal at the base of the Sydney Basin and exposure of underlying Devonian shale, siltstone or quartzite. Eastern margin of Narrabeen Group sandstone in cliffs. Small areas of hill top Paleogene and Neogene basalt.

Wide valleys, low-rolling hills below sandstone cliffs, Isolated flat top mountains in the valleys formed as pinnacles or remnant pieces of plateau. Steep, bouldery debris slope below cliffs. Shoulder slopes with stone pillars or "pagodas" above steep canyons on tributary streams falling into gorges. Low gradient swampy stream lines.

Shallow stony texture contrast profiles, usually with gritty well drained A horizons, over tough yellow or grey poorly drained clays. Bouldery debris with clay matrix below cliffs (talus). Organic sands in swamps. Red brown structured loams on basalts.


Triassic Wianamatta Group shales and sandstones. A downwarped block on the coastal side of the Lapstone monocline. Intruded by a small number of volcanic vents and partly covered by Paleogene and Neogene river gravels and sands. Quaternary alluvium along the mains streams.

Low rolling hills and wide valleys in a rain shadow area below the Blue Mountains. At least three terrace levels evident in the gravel splays. Volcanics from low hills in the shale landscapes. Swamps and lagoons on the floodplain of the Nepean River.

Red and yellow texture contrast soils on slopes, becoming harsher and sometimes affected by salt in tributary valley floors. Pedal uniform red to brown clays on volcanics. Poor uniform stony soils, often with texture contrast profiles on older gravels, high quality loams on modern floodplain alluvium.


Permian quartz sandstone and mixed shale and lithic sandstones. Paleogene and Neogene trachyte intrusives at Milton. Limited Paleogene and Neogene sands and more extensive Quaternary coastal sands.

Escarpment faces west and south and sandstone plateau rises to small peaks like Pigeon House. Waterfalls and gorges off the escarpment but low hills and coastal ramp on siltstones to Jervis Bay. Well-developed coastal barrier with Jervis Bay enclosed by tied islands. Pleistocene cliff top dunes on the peninsula with fresh lakes created by watertable windows.

Poor shallow sands on quartz sandstone plateau similar to Ettrema. Deep texture contrast soils with loam topsoils on coastal shales, moderate fertility but waterlogged valley floors. Coastal barriers extend from clean dune sands to deep podzols in Pleistocene dunes. Organic sands and muds in swamps and estuary.

aInterim Biogeographic Regionalisation for Australia (SEWPaC, 2012)

bsource document does not differentiate between Capertee Upland and Capertee Valley

Data: NSW National Parks and Wildlife Service (2003)

Soils of the Sydney Basin bioregion can be classed into seven main Australian Soil Classification (Isbell, 2002) soil types (Figure 12): Tenosols (25%), Kurosols (22%), Kandosols (16%), Rudosols (15%), Dermosols (10%), Sodosols (6%) and Chromosols (4.0 %). Localised occurrences of Ferrosols, Hydrosols, Podosols and Organosols have also been mapped. The Hawkesbury-Shoalhaven physiograpic province is dominated by Tenosols, Dermosols and Kandosols in the north and west, with Kurosols, Rudosols and Ferrosols becoming more important to the south; the Cumberland Lowland province is characterised by Sodosols and Kurosols; that part of the Monaro Fall province that lies within the Sydney Basin bioregion is dominated by Kurosols; and the Bathurst Tablelands province, which includes the Capertee Uplands and Capertee Valley IBRA subregions, has a wide mix of soil types, including significant areas of Chromosols and Sodosols. Characteristics of the main soil types are summarised below, ordered by descending area.

Tenosols are soils which are young, typically very sandy and exhibit a weakly developed soil profile, except perhaps for the A horizon. They generally have low agricultural potential due to low chemical fertility, poor structure and low water holding capacity.

Kurosols tend to develop in mid- and lower slope positions. They are characterised by a clear, sharp textural boundary between coarser textured A horizons (e.g. sands or loams) and finer textured (i.e. clayey) B horizons (Isbell, 2002). The surface of the soils is often acidic (pH <5.5). These soils are found predominantly in the Cumberland and Monaro Fall IBRA subregions.

Kandosols do not exhibit strong texture contrast between the A and B horizons. Instead the B horizon tends to have a massive or weakly developed structure. They have moderate chemical fertility and water holding capacity.

Rudosols are generally associated with upper slopes, ridges and crests. These soils are poorly developed and typically young, so have had little time to develop structure. They may be deep or shallow, and either clayey, or loamy or sandy throughout the profile. Rudosols may also be stony.

Dermosols are likely to be dominated by clay that is near uniform to slowly changing in texture in the profile (Isbell, 2002). These are well-structured soils and generally more clayey in the floodplains, where the deepest soils in the Sydney Basin bioregion are likely to be found. They occur in the lower lying parts of the Hawkesbury-Shoalhaven Plateau, north and east of Lithgow, and along the coast to the south of Wollongong.

Sodosols generally develop on lower hillslopes or in perched upper slope locations. They are often associated with salinity (e.g. at seeps or where drainage is poor). Soils exhibit a strong contrast in textures between topsoil and poorly structured, dispersible clay subsoils. Agriculture can be a challenge due to structural issues (caused by excessive sodium ions) and salinity. In the Sydney Basin bioregion, Sodosols occur in the western parts of the Cumberland IBRA subregion, east of the Nepean River and in the north-west corner of the Capertee Valley and Capertee Uplands IBRA subregions.

Chromosols have a strong texture contrast and form from quartz-rich parent material. In the Sydney Basin bioregion, they are found in the upper Capertee river basin and in the Nepean river basin in the south of the Cumberland Lowlands physiographic province.

Organic rich soils are typical of the bioregion’s many swamps and estuarine areas.

Figure 12

Figure 12 Australian Soil Classification (ASC) classes for the Sydney Basin bioregion

Data: CSIRO (Dataset 8) Vegetation cover

Figure 13 shows the land cover in the Sydney Basin bioregion in 2008, based on remotely sensed data that are post-processed to convert vegetation greenness to a land cover type (Geoscience Australia, Dataset 9). The bioregion is overwhelmingly dominated by trees, reflecting (i) good rainfall, (ii) the rugged topography (i.e. difficult to develop) of the deeply incised sandstone country of the Blue Mountains and Illawarra and Southern Highlands escarpments, and (iii) the associated conservation status of significant tracts of the bioregion and the protected area status of much of the water supply catchments around Sydney and Wollongong (see also Figure 15). Major vegetation type mapping of the bioregion (Figure 48 in Section 1.1.7) indicates that these treed areas are mostly different eucalypt forest and woodland types, but that the lower density treed areas of the Cumberland IBRA subregion around Sydney include cleared, non-native vegetation and buildings. Areas of irrigated pasture are evident along the coastal zone between Wollongong and Nowra, with much of the rest of the land cover being classed as rainfed pasture.

Figure 13

Figure 13 Vegetation cover of the Sydney Basin bioregion

Data: Geoscience Australia (Dataset 9)

Last updated:
21 January 2019
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