The Mandalong valley consists of a broad floodplain surrounded by the Watagans to the west and south and by a separated ridge complex to the south east. The surrounding hills and valleys form the catchment for rainfall that then flows north east through the valley by way of meandering streams and flood ways and ultimately into Lake Macquarie through Stockton and Dora Creeks.
The area of the Mandalong Valley catchment including the valley floor is just over 5000ha.
About 600ha of the valley floor lies within the bounds of the 1:100 year flood limit and because the area is very flat and low-lying (average elevation 14m; average slope 20) there are areas of pooling which form wetlands.