The highway is part of the Great Inland Way linking Sydney and Cairns, and provides all-weather access to rugged black opal country of Lightning Ridge.

It continues in a south-easterly direction through Dunedoo, sharing a short concurrency with Golden Highway, continues through the regional centres of Gulgong and Mudgee, through Ilford and the junctions of Bathurst-Ilford Road and Bylong Valley Way, before eventually terminating at an interchange with Great Western Highway at Marrangaroo, 10 kilometres (6.2 mi) north-west of Lithgow.

[11][12] The alignment north of Lightning Ridge was altered on 17 June 1959 to meet the state border with Queensland at Hebel via Angledool, instead of via Brenda.

[13] In December 1982, a 15 kilometre deviation opened south of Mudgee as part of the Windamere Dam project.

Queensland however signed Carnarvon Highway as National Route 55 between 1983 and 1997 rather than Castlereagh Highway, causing a major discrepancy for many years in that National Route 55 met the border at Hebel but abruptly begun again over 100 kilometres (62 mi) east along the border at Mungindi.

The Queensland Road Department changed this in 1997 and signed National Route 55 over the border north to St George, and eventually to Charters Towers in later years.