Within Victoria, the passing of the
Country Roads Act 1912 through the
Parliament of Victoria provided for the establishment of the
Country Roads Board (later
VicRoads) and their ability to declare Main Roads, taking responsibility for the management, construction and care of the state's major roads from local municipalities.
Ouyen-Pinnaroo Road was declared a Main Road from
Ouyen via
Walpeup and
Murrayville to the state border with
South Australia on 14 December 1914, The passing of the
Developmental Roads Act 1918 allowed the Country Road Board to declare Developmental Roads, serving to develop any area of land by providing access to a railway station for primary producers.
Ouyen-(Kulwin-)Manangatang Road was declared a Developmental Road, between Ouyen and
Kulwin on 8 April 1920, and between Kulwin and
Manangatang on 18 November 1920 The passing of the
Highways and Vehicles Act 1924 provided for the declaration of State Highways, roads two-thirds financed by the State government through the Country Roads Board.
Ouyen Highway was declared a State Highway within Victoria in the 1947/48 financial year, the highway was renamed as
Mallee Highway, and extended east along the former
Ouyen–Piangil Road to
Piangil in December 1990, subsuming the original declarations of Tooleybuc Road as a Main Road and Ouyen-Manangatang Road as a Developmental Road. The highway was signed as National Route 12 between
Tailem Bend and
Ouyen in 1955, later extended with the road to Piangil in 1990. With both states' conversion to their newer alphanumeric systems in the late 1990s, its former route number was updated to B12 in 1997 (within Victoria), and in 1998 (in South Australia). The passing of the
Road Management Act 2004 granted the responsibility of overall management and development of Victoria's major arterial roads to
VicRoads: in 2004, VicRoads re-declared the road as
Mallee Highway (Arterial #6650), beginning at the South Australian border at
Panitya and ending at the New South Wales border in
Piangil. ==Major intersections and towns==