The Australian Third Division Telegraphists' and Postal Clerks' Union had its origins in the Australian Telegraphists' Union formed in 1920 as a breakaway group of the Australian Commonwealth Post and Telegraph Officers' Association. It deregistered just a few months later but was formed again in 1921 under the same title. By 1925 it was known as the Australian Telegraphists' and Clerical Assistants' Union. Two years later, in 1927, the Union re-emerged as the Australian Third Division Telegraphists' and Postal Clerks' Union, the term 'Third Division' referring to the clerical division of the Commonwealth Public Service. Operating until 1966, the Third Division Telegraphists' and Postal Clerks' Union reregistered as the Union of Postal Clerks and Telegraphists and the Fourth Division Postmasters, Postal Clerks and Telegraphists' Union merged with it, effective from 19 February 1967.

The Australian Commonwealth Post and Telegraph Officers' Association was formed in 1912, and changed its name to the Australian Postal Assistants' Union in 1917. In 1926 it was renamed the Fourth Division Postmasters', Postal Clerks' and Telegraphists' Union, the term 'Fourth Division' referring to the lowest-paid division of the Commonwealth Public Service. It remained an autonomous body until amalgamating with the Australian Third Division Telegraphists' and Postal Clerks' Union, effective from 19 February 1967, to form the Union of Postal Clerks and Telegraphists.