The External Communications function of the Australian Government was passed from the Prime Ministers Department (CA 12) to the Department of External Affairs [II] (CA 18) by a Prime Ministerial directive dated 21 May 1943. Both departments were then housed in West Block and the two cable sections were amalgamated from 17 May into the External Communications Branch of External Affairs. The Prime Minister directed that, for reasons of security and economy, all overseas cables to and from the Australian Government were to be handled by External Affairs.

Transmission overseas of messages from all departments involved various cyphering methods and a variety of cable lines. In the early years, even encrypted cables were sent through public channels. Eventually, most cables were sent on some form of teleprinter.

Printed copies were duplicated onto pre-printed cable forms which carried a security classification vertically down the right hand side of the page in different coloured inks. After the required number of copies had been run off, the master sheets (as the stencils were called) were filed in chronological/numerical order by bundle. Outward master sheets for 1939-1949 are held as CRS A3196.

 Most Australian overseas missions were set up after the end of World War II. By 1950, cable traffic had increased to the point where it was more efficient to keep inward and outward sets of the printed copies in binders by post, in post serial number order. From January 1950, master sheets were kept only for administrative purposes and not as an archival record.

Post serial numbering ensures that any gaps in traffic between Canberra and a post would be easily noted. Outward cables from Canberra to each post were numbered sequentially, as well as carrying a message reference number, and post cables sent in response were also numbered sequentially. This number was placed in the centre of the page above the body of the text and was the number generally referred to in subsequent correspondence. Registers linking post and cable serial numbers were kept solely for the short term purpose of ensuring that all cables sent had been received. They carried no other identifying information and have not survived. In some cases, the range of the post serial numbers is written on the spine of the binder.

Savingrams (non urgent cable messages sent in off peak time or by diplomatic bag) were filed in a separate run of binders and duplicated onto special forms printed in green. Later, they were known as BB (by bag) cables, ranking lower than Routine Priority (RR).

Until about 1961, outgoing cables were registered in large bound volumes (CRS A3643). Entries are chronological and show; from, addressee, outward cable reference, check initial, inward cable reference, outward cable reference, number of words and subject.

From 1939 until May 1974, outgoing cables were given serial numbers with an O (Outward) prefix. Numbers started from 1 at the beginning of each new calendar year. The introduction of computerisation in 1974 made a radical change to the numbering system. All cable messages henceforth were outward, either from Canberra or from the transmitting post, represented by a bigram (sometime a tetragram). Under the new system, serial numbering for each post was continuous until the number reached 999999, when it cycled back to 1. Rather than disrupt the system of keeping printed cables by year, it was decided to keep cables from January to May with the rest of those for 1974.