The Namoi River is one of the Murray-Darling Basin's major NSW sub-catch merits. It covers an area of about 42,000 km from the Great Dividing Range near Tamworth to the Barwon River near Walgett. The Peel River is a major regulated tributary to the Namoi with a catchment area of around 4,700 km. It contributes an average annual volume of approximately 280,000 ML to the Namoi River.

The Namoi catchment borders the Gwydir and Castlereagh catchments and is bounded by the Great Dividing Range in the east, the Liverpool Ranges and Warrumbungle Ranges in the south, and the Nandewar Ranges and Mount Kaputar to the north. Elevations range from over 1,500m to the south and east to just 100m on the alluvial floodplain of the lower catchment west of Narrabri. Stretching from Bendemeer in the east to Walgett on the western boundary, the Namoi catchment is over 350 km long.

Major tributaries of the Namoi River include Coxs Creek and the Mooki, Peel, Manilla, and McDonald Rivers joining the Namoi upstream of Boggabri with Plan, Narrabri, Baradine and Bohena Creeks joining below Boggabri. Major tributaries of the Peel River are Goonoo Goonoo Creek, Cockburn River, and Dungowan Creek. Streamflows in the Namoi catchment are regulated by Keepit Dam on the Namoi River, Split Rock Dam on the Manilla River and Chaffey Dam on the Peel River. The regulated section of the Peel River has historically been managed as a separate allocation scheme to the Namoi regulated river, and operationally the management of Chaffey Dam is independent of the other storages on the Namoi.