The Mid Brunswick catchment lies predominantly on the Swan Coastal Plain and consists of land draining to the Brunswick River downstream of South West Highway, and part of the Wellesley River downstream of Devlin Road. The lower reaches of the Brunswick are tidally influenced and mix with estuary water.

To the east of the Wellesley River most of the Mid Brunswick catchment has been cleared for stock grazing and is heavily irrigated. To the west of the Wellesley River large areas of undisturbed vegetation remains, including the ecologically important Kemerton Wetlands. Sandy soils and irrigation in the catchment mean that fertiliser and animal waste can easily leach into tributaries and drains, and eventually the river. Grazing of cattle, dairies and the Brunswick wastewater treatment plant all contribute a substantial nutrient load to the river and estuary.