Australian railway telegraphic codes were originally devised to reduce the size of telegraphic messages, though some have survived into the telephone era. They were basically used in telegrams between locations in a railway network of offices, stations and other parts of the railway system.
The distinction between telegraphic code, and telegraphic code addresses is an important one to make. Many businesses identified on their stationery their telegraphic address, as well as their telephone number.
In some states, railway operations would have offices with abbreviated addresses.
The codes were devised with many features common to all states, with states devising additional codes for their own use if required. The codes were listed either on a large poster or in a telegraph code book.
Each code might consist of four letters, in two syllables, with a two letter difference to any other code. Some codes expected additional numbersand/or words as parameters. Codes are grouped by category and the first letter(s) of the codes in each group are the same.
More information on the Wikipedia page  which gives some examples.