[4] The main purpose was to house offices for the Treasury Department, along with rooms for the Chief Secretary, the Treasurer, the Registrar-General, the Registrar of the Supreme Court, and the Auditor General, as well as one for the Governor of Victoria when he was on government business.

Construction of the basement and ground floor began in 1858, when he took seven months leave to tour Europe and the United Kingdom.

On his return he completed the design, possibly changing that of the second floor, and removing the fourth level attic of the central section in favour of a continuous eave.

In 1868 these were replaced with the grand forecourt and cast iron lamps still standing today, also designed by Clark.

The building is also notable for its role when Melbourne was the temporary capital city of Australia after Federation, with the National Executive Council meeting there.

[1] He then also proposed to the Executive Committee of Victoria's 150th Anniversary Celebrations in 1984 that a new Museum of Social and Political History be established at the Old Treasury Building.

The Old Treasury was built to house the Treasury Department and store gold, but also provided offices for the leaders of the young colony, including the Governor, the Premier (at the time called Chief Secretary), the Treasurer, the Registrar-General, the Registrar of the Supreme Court and the Auditor General.