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 operates the signals interception and monitoring facility – known locally as "the house with no windows" – on Australia's Indian Ocean territory, 1100 kilometres south-west of Java.

Never publicly acknowledged: The Cocos Island facility. Credit:Leigh Henningham

The station has never been publicly acknowledged by the Australian government, or reported in the media, despite being in operation for more than two decades.

Along with the better-known Shoal Bay Receiving Station near Darwin, the previously unreported Cocos Island facility forms a crucial part of Australia's signals intelligence collection efforts targeting Indonesia.