In Australia, the normal protocol for radio station call-signs is a number (identifying the state or territory, as per below) accompanied by either a two-letter (AM) or three-letter (FM) call-sign.
Of course, there are plenty of exceptions to the rule - For example: Brisbane radio station 4TAB despite it being an AM-band station; and the national broadcaster ABC identifies some regional stations with nABCRR (Regional Radio) or nABCRN (Radio National).
The number prefixes for call-signs are assigned as follows:
1 - Australian Capital Territory
2 - New South Wales
3 - Victoria
4 - Queensland
5 - South Australia
6 - Western Australia
7 - Tasmania
8 - Northern Territory
The Northern Territory was initially assigned the 5 prefix (with South Australia) but later was assigned 8. The Australian Capital Territory was initially assigned the 2 prefix (with New South Wales) but this was later changed to 1 for new stations or existing stations that chose to adopt a new call-sign.
The letter combinations assigned to radio stations can be derived from a number of sources - sometimes it can reflect the station's original ownership (e.g. 3AK in Melbourne was named after the Akron Tyre Company), reflect the station's location (e.g. 3MEL in Melbourne or 2CA in Canberra) or coverage area (e.g. 3GG 'Greater Gippsland') or indeed can have no significance at all (e.g. 3XY and 3UZ in Melbourne).
In the 1970s and '80s it became popular for new radio stations to adopt 'double' or 'triple' letter call-signs, e.g. 5AA in Adelaide, 2CC in Canberra, 2MMM in Sydney or 8DDD in Darwin. Some existing AM stations also chose to change their original call-signs to the 'double' format, e.g. 3DB in Melbourne became 3TT, 3UL in Warragul became 3GG, and 3XY in Melbourne became 3EE.
The advent of FM radio saw many stations adopt words as their three-letter call-signs, e.g. 3EON and 3FOX in Melbourne, 2NEW in Newcastle, 2KIX in Canberra and 3BAY in Geelong, while Sydney's 2DAY went a step further and pronounces its whole call-sign as "Today".
Commercial stations converting from AM to FM often just duplicated a letter to their existing call-sign - e.g. 3KZ in Melbourne became 3KKZ, 6PM in Perth became 6PPM, and 7HO in Hobart became 7HHO.
