The Thomson Brook catchment lies on the Darling Plateau. It consists of land draining to the Brook upstream of the gauging station near Thomson Brook Road, approximately 1.5 km west of Forrest Road.

Around two thirds of the upper catchment is uncleared native vegetation, with parts of the lower catchment used for stock grazing and plantation timber. The soils in the area have a relatively high capacity to retain nutrients, and in combination with the large areas of uncleared native vegetation, this results in better water quality in comparison to catchments further downstream.