The Wellesley catchment lies on the Swan Coastal Plain and consists of land draining to the Wellesley River, Mangosteen Drain and Mornington Creek. Benger Swamp Nature Reserve is located in the centre of the catchment with Myalup State Forest in the west. The Wellesley River drains to the Brunswick River, ultimately contributing flow and pollutants to the Leschenault Estuary.
Most of the catchment has been cleared and is extensively irrigated and drained to support the beef and dairy industries. This grazing land is on sandy soils which have a low ability to retain phosphorus. Together with a number of dairy sheds, this is a substantial source of nutrients to the river. The combination of nutrient intensive land uses, poor soils and regular irrigation mean that the Wellesley catchment is a major source of nutrient pollution to the Leschenault Estuary. This is confirmed by very high measured concentrations of nutrients in rivers and drains in the catchment.