Following the election of a Labor government to office in October 1911, legislation was introduced to amalgamate the various water supplies throughout the State. The new Department assumed responsibilities for the following activities on 1 August 1912:

• the Goldfields Water Supply undertaking;
• the Metropolitan Water Supply, Sewerage and Drainage undertaking;
• Agricultural and Country Town water supplies, Irrigation and drainage, artesian boring;
• water supplies, stock routes in Mining Districts.

(North West water supplies continued to be managed by the Engineer for the North West in the Public Works Department)

The Under-Secretary of the Department was H.C. Trethowan and the two joint engineering heads were Hugh Oldham for Metropolitan and Agricultural areas and P.V. O'Brien for Goldfields and Northern Areas.

During the life of the Department, projects of major importance included the overhaul of the Coolgardie water supply pipeline and the beginning of the irrigation scheme at Harvey.

In 1914 the Metropolitan region was expanded to include Armadale, Guildford and Midland. During that year Perth, Fremantle and Claremont were consolidated into a central district. During 1916 and 1917 local councillors and Liberal politicians pressured the Government to reinstate Board control for the Metropolitan concerns. Between April and June 1918, the latter's functions were split administratively from the rest of the Department. Both organisations remained part of the same Department until 1921. In that year the MWSS & DD was established and the Public Works Department assumed formal control of the country operations.

This organisation was established in 1910 pursuant to Statute No. 43 of 1909. The Act consolidated water supply, sewerage and drainage schemes under one administration, directly responsible to the Minister of Water Supply, Sewerage and Drainage.

Between August 1912 to April 1918 the Department was administered by and integrated with the Water Supply, Sewerage and Drainage Department (MWSS & DD). The MWSS & DD's status as a separate Department was not re-established until July 1921.

In October 1925 the Engineering staff of the Department come once more under the direct supervision of the Public Works Department's Chief Engineer. There was thereafter a pooling of staff resources, and accommodation between the two Departments. Three water supply, sewerage and drainage districts were gazetted as constituting the Metropolitan Area in January 1910. These districts were Perth, Claremont and Fremantle where water supply and sewerage schemes already existed. In 1914 Guildford, Midland and Armadale were also included in the Metropolitan Area.

In September 1963 the Liberal Government introduced a Metropolitan Water Supply Sewerage and Drainage Amendment Bill with the intention of substituting Board for Departmental control. The Act was promulgated in November 1963 and a new administrative structure established by July 1964.