Gulargambone is a small town in the central-west plains of New South Wales, Australia, on the banks of the Castlereagh River, in Coonamble Shire. It is 382 km north-west of Sydney. At the 2016 census, Gulargambone had a population of 400.

The Castlereagh River is located in the central–western district of New South Wales, Australia. It is part of the Macquarie-Castlereagh catchment within the Murray–Darling basin and is an unregulated river, meaning no dams or storage have been built on it to control flows. On a map of NSW, the Castlereagh has a distinctive appearance among the north-western rivers for its fish-hook-like shape: from upstream in the north at its confluence with the Macquarie River it extends southwards to a hook-shape, flattened-out at the base, which curves to the right through to the tip of the hook in the Warrumbungle Mountains at the river's source.