[2] The Castlereagh Nature Reserve contains a geographically restricted substrate of Tertiary alluvials producing clays, sand, gravel and shales at depth.

The nature reserve is between 30 and 40 metres (98 and 131 ft) above sea level and is predominantly flat with broad, shallow depressions forming a sparse drainage network.

Threatened plant species found in the nature reserve include Dillwynia tenuifolia, Pultenea parviflora, Acacia bynoeana, dwarf casuarina Allocasuarina glareicola, nodding geebung Persoonia nutans and Micromyrtus minutiflora.

Heavy harvesting of ironbark during the Second World War, illegal timber collection and numerous fires generally thinned the forest, resulting in no mature trees in some areas.

The nature reserve is home to a diverse range of mammals including sugar gliders, Petaurus breviceps; brush-tailed possum, Trichosurus vulpecula; and eastern grey kangaroo, Macropus giganteus.