Castlereagh Highway

Castlereagh Highway is a 790-kilometre (490 mi) state highway located in New South Wales and Queensland, Australia. From north to south the highway traverses South West Queensland and the North West Slopes, Orana, and Central West regions of New South Wales.

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.

Castlereagh Highway was named after the Castlereagh River, which it parallels for most of its journey north from Gilgandra.

Castlereagh Highway commences at an intersection with Carnarvon Highway, south of St George, Queensland, and heads in a south-westerly direction via Dirranbandi to Hebel, where it crosses the state border with New South Wales just beyond and continues in a southerly direction, past Lightning Ridge to Walgett, where it crosses the Barwon and Namoi Rivers and meets the Gwydir and Kamilaroi Highways. Continuing south through Coonamble, the highway crosses the Castlereagh River and meets the Oxley and Newell Highways in Gilgandra. 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.

The passing of the Main Roads Act of 1924 through the Parliament of New South Wales provided for the declaration of Main Roads, roads partially funded by the State government through the Main Roads Board (MRB, later Transport for NSW). Main Road No. 55 was declared from the intersection with Great Western Highway near Marrangaroo, via Ilford, Mudgee, and Gulgong to Craboon (and continuing northwards via Coolah eventually to the intersection with Oxley Highway at Mullaley), Main Road No. 56 was declared from Walgett, via Coonamble to Gilgandra (and continuing southwards via Dubbo, Forbes, Cowra, Yass and Canberra eventually to the intersection with Queanbeyan-Braidwood Road, today Kings Highway, at Queanbeyan), and Main Road No. 205 was declared from Craboon via Dunedoo to Mendooran (and continuing northwards via Tooraweenah to Coonamble), on the same day, 8 August 1928. With the passing of the Main Roads (Amendment) Act of 1929 to provide for additional declarations of State Highways and Trunk Roads, these were amended to Trunk Roads 55 and 56 and Main Road 205 on 8 April 1929. Trunk Road 77 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 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 & 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.