Todd and his wife, Alice, arrived in South Australia in November 1855. Todd's earliest achievements in the colony were the completion of the first government telegraph between Adelaide and its port in 1856 and the connection of Adelaide and Melbourne by telegraph - completed 1858. Adelaide and Sydney were connected via Melbourne shortly after this. A direct line from Adelaide to Sydney was completed, also under Todd's direction, in 1866. Todd was involved in the establishment of the Adelaide Observatory (on West Terrace) in 1860. He also established a system of relaying meteorological observations by telegraph throughout South Australia and the Northern Territory.

In 1870 Todd was made postmaster-general and superintendent of telegraphs and managed to convince the South Australian government to agree to a scheme that he had been advocating since 1859 - that of a transcontinental telegraph which could potentially be used to connect Australia to Europe via Asia. Much of the continent's interior remained unexplored, but McDouall Stuart's successful crossing of Australia from south to north in 1861-62 convinced Todd that a cable could be laid. The line was completed in August 1872 and cable communication began in earnest a couple of months later. The Overland Telegraph connected Port Augusta to Darwin, where it joined an underwater cable to Java, which linked Asia and Europe.

Todd served the South Australian government for 50 years and his service was so valued that the parliament chose not to pass a Bill for compulsory retirement at 70 until after Todd had chosen to retire.