I managed to grab a photo of one of the various public address installations dotted around Sydney CBD and they were all (that I saw) mounted on traffic lights.
This is one on the corner of Castlereagh and Park streets, just up from the hotel I was staying at and has "Bosch" printed on the horn and super-imposed on the back of the driver enclosure (with some serious cooling fins I might add). They now remain as a permanant public address system for Sydney CBD.
Here's an image of it, uploaded to Wikipedia for everyone's benefit. Before that image, there was nothing that I could find (at least of legible resolution).
http://en.wikipedia.org/wiki/Image:Apecpas20071115.jpg