Signalling was initially done using flags, however by 1831, a three-armed semaphore (an upright post with arms) was operating which was capable of dealing with 666 code signals. This was replaced in 1838 by a six-armed semaphore over 24 metres high which could handle over 900 000 separate signals.
In 1836 the station was linked into the Tasman Peninsula system. Through this network of semaphore stations, messages could be rapidly relayed from the penal settlement at Port Arthur to Hobart. The Hobart semaphore is located at the Mulgrave Battery in Castray Esplanade in Battery Point.
Semaphore messages were sent by raising and lowering the arms to set positions. A 20 word message could be sent from Hobart and received at Port Arthur in 15 minutes. Three signal men usually worked and lived on site, often accompanied by their families. Work was carried out in all weather conditions on watches of three hours from 6am to 9pm.
The signal station was operated by the military from its inception until 1858, when the newly established Marine Board of Hobart took over control. By 1860, the station had developed into a two-tiered system, with the top six arm semaphore dealing with Port Arthur maritime traffic, while the lower arm dealt with Hobart traffic.
The change to telephone
In 1880, a mere four years after the invention of the telephone by Alexander Graham Bell, a telephone line - Tasmania's first - was established between Hobart and Mount Nelson. The arrival of this new technology resulted in the the removal of the semaphore and its replacement with a flagstaff - but not before the semaphore sent one last message (no. 343 which means "forgotten").
In 1958 a base station for ship-to-shore radio-telephone, the second of its kind in Australia, was installed at Mount Nelson. The Mount Nelson Signal Station ceased operations in 1969, ending 158 years of Tasmanian communications history.
Management of the Mount Nelson site was transferred from the Marine Board to the Parks and Wildlife Service in 1979.
Today, the station retains a strong link with the past. Every day, the Tasmanian State flag is flown, and International Marine Signal Flags are used to welcome visiting ships and for other special occasions.