The wetlands of Ermington Bay are one of eight significant remnant wetlands listed on the Register (Meadowbank Bay/Mud Flats, Yarralla Bay, Majors Bay, Mason Park, Homebush Bay, Lower Duck River and Haslem's Creek). These were once part of an extensive wetland system bordering the Parramatta River. Mangroves of the Parramatta River area represent a significant proportion of the mangroves remaining in the Sydney Region. The uncommon species WILSONIA BACKHOUSEI occurs at Ermington Bay. The wetlands of the Upper Parramatta River provide habitat for a diverse bird community; seventy-five species have been recorded of which thirty-seven species occur regularly. It also provides breeding habitat for seventeen species. The remnant wetlands of the Upper Parramatta River have been ranked sixth in importance for waders in New South Wales and they provide habitat for a large number of species listed in the Japan Australia Migratory Birds Agreement (JAMBA), twenty species and, China Australia Migratory Birds Agreement (CAMBA) nineteen species. Two species, which occur in the area, the little tern (STERNA ALBIFRONS) and the black tailed godwit (LIMOSA LIMOSA) are listed under Schedule 12 (Endangered Fauna) of the NSW National Parks and Wildlife Act (1974). The remnant wetlands also support one of the two Sydney colonies of the white fronted chat (EPHTHIANURA ALBIFRONS) and contribute habitat for one of the largest populations of chestnut teal (ANAS CASTANEA) in NSW.
The soils of Ermington Bay are predominantly stream alluvium and estuarine sediments, including silts and clays derived from sedimentary parent material of Triassic origin, particularly the Wianamatta Group shales. Ermington Bay includes areas reclaimed with artificial fill of dredged estuarine sand and mud, demolition rubble, waste materials, rock and local soil material. Stands of grey mangrove (AVICENNIA MARINA) line the river and intertidal mudflats. The saltmarsh area has been reduced to a fringe between the mangroves and the higher embankments of reclaimed areas, with an expanded area at the western end of the wetlands in which WILSONIA BACKHOUSEI occurs. The intertidal mudflats are important estuarine areas for both wading and non-wading birds.