How quaint!
I note that the ‘unit of competency’ called ‘maintain aircraft radio communications’ for aviation-related diplomas published on the training.gov.au website lists as ‘assessment requirements’ knowledge of ‘responsibilities of an aeronautical radio operator, including secrecy of communications and unauthorised transmissions.’
Methinks the secrecy bit is based on folklore rather than existing law, at least in relation to the content of civil aircraft-related radiocomms. I could be wrong. But it would be a very frequently breached law!