was later declared from the intersection with Oxley Highway at Gilgandra via Mendooran and Dunedoo, to Trunk Road 55 at Craboon, on 16 March 1938. The Department of Main Roads, which had succeeded the New South Wales MRB in 1932, declared State Highway 18 on 16 March 1938, from the intersection with State Highway 11 (later known as Oxley Highway) at Gilgandra via Coonamble and Walgett to the state border with Queensland near Brenda; with the declaration of State Highway 17 (later known as Newell Highway) on the same day, the northern end of Trunk Road 56 was truncated to meet State Highway 17 at Forbes, as a result. State Highway 18 was named Castlereagh Highway on 12 May 1954, after the Castlereagh River, with the river named in honour of Lord Castlereagh. 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. In December 1982, a 15 kilometre deviation opened south of Mudgee as part of the Windamere Dam project. The Roads and Traffic Authority, succeeding the New South Wales DMR in 1989, extended the highway declaration south from Gilgandra via Dunedoo and Mudgee to Marrangaroo on 3 October 1997, subsuming Main Road 77 (previously Trunk Road 77). The southern end of Main Road 56 (previously Trunk Road 56) was truncated to meet State Highway 18 east of Craboon, as a result. The passing of the Roads Act of 1993 through the Parliament of New South Wales updated road classifications and the way they could be declared within New South Wales. Under this act, Castlereagh Highway today retains its declaration as Highway 18, from the state border with Queensland at Hebel to the intersection with Oxley Highway in Gilgrandra, then from the intersection with Newell Highway in Gilgandra to the intersection with Golden Highway in Dunedoo, then from the intersection with Golden Highway near Craboon to the interchange with Great Western Highway at Marrangaroo. Within New South Wales, the highway was signed National Route 55 in 1974 from Gilgandra to Walgett, and State Route 86 south of Gilgandra; National Route 55 was extended north from Walgett to the border with Queensland in 1983. 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