The Peel-Harvey Estuary is the dominant geographic feature of Mandurah and the largest estuary in southwestern Australia — roughly twice the size of Sydney Harbour. The water is calm, shallow, and brackish, and the catchment supports a large population of black swans, pelicans, ibis, and migratory wading birds, along with the resident dolphins.
The estuary was nutrient-overloaded for decades by farm runoff, until a six-kilometre artificial channel called the Dawesville Cut was opened in 1994 to flush the system with ocean water. The estuary's health has been steadily improving since. Most of the foreshore is now public reserve, and walking and cycling trails run for tens of kilometres along its edge.