OK so probably one in ~Darwin (because gateway hub for Pine Gap), one in ~Canberra (because capital) and one in ~Adelaide (because center of the industry).
Haha. I'm sure any intel agency worth its salt can do better than my supposition. Sleeping on it, I realised that since Singapore is presumably a major five eyes sigint hub there is likely an additional attraction to Darwin re. reduced latency access and reduced transit costs for newly captured undersea cable data from Asia.