Australia's electronic spy agency is intercepting Indonesian naval and military communications through a secret radio listening post on the remote Cocos Islands.
According to former defence officials, the Defence Signals Directorate runs the signals interception and monitoring base on Australia's Indian Ocean territory, 1100 kilometres south-west of Java.
Along with the better-known Shoal Bay Receiving Station near Darwin, the previously unreported Cocos Island facility forms a key part of Australia's signals intelligence efforts targeting Indonesia.
Known locally as ''the house without windows'', it includes radio monitoring and direction-finding equipment and a satellite ground station. But the station is of little help in combating people smuggling, according to the former intelligence officers.
The station has never been publicly acknowledged by the government, nor previously reported in the media, despite operating for more than two decades.
The Defence Department would not comment, and said only that it hosts ''a communications station'' that formed part of the wider defence communications network.
But former defence officers have confirmed that the station is a Defence Signals Directorate facility devoted to maritime and military surveillance, especially Indonesian naval, air force and military communications.
Google Earth imagery of the property, discreetly placed amid coconut palm groves on the south-east part of West Island, shows four cleared areas each with radio mast sets, including a 44-metre-wide ''circularly disposed antenna array'' for high-frequency and very high-frequency radio direction finding.
Australian National University intelligence expert Des Ball said the facility was operated remotely from the Defence Signals Directorate headquarters at Russel Hill, in Canberra. Intercepted signals are encrypted and relayed to Canberra.
He said preparations for the Cocos station began in the late 1980s, and involved a highly secretive signals intelligence group, the Royal Australian Air Force's No. 3 Telecommunications Unit.
In the face of what it described as ''extremely challenging logistics'', an Adelaide-based company, Australian Satellite Communication, then installed a communications satellite earth station at the facility.
The Cocos Island signals intelligence station forms part of broad Australian espionage efforts directed at the Indonesian government.
As reported by Fairfax Media on Thursday, these programs include a covert Defence Signals Directorate surveillance facility at the Australian embassy in Jakarta. One former defence intelligence officer said Australia's monitoring of Indonesian communications was ''very effective'' and allowed assessments of the seriousness of Indonesian efforts to combat people smuggling.
But the former intelligence officer said the Cocos and Shoal Bay facilities were of ''limited utility'' in finding vessels carrying asylum seekers that avoided using radios or satellite phones until they contacted the Australian Maritime Safety Authority.
Richard Tanter, of the Nautilus Institute of Security and Sustainability, said the Cocos Islands station was likely to be intercepting increasing volumes of naval and military communications.
''With the increasing Australian and US interest in the Indian Ocean region, it is likely to become more important,'' he said.