While no image is available, direct observations were made of flies doing something to the egg sac.
(Converting from comment)
Based on internet searches, it appears likely that the fly is Sarcophaga arachnivora, which lives off of spider egg cases in its juvenile form; the fly is laying eggs.
If you follow that link you will see references to a couple of papers that describe the species in more detail, though I don't believe I have access to them.
The geographic location noted there of SE Australia agrees with your observation of the organism.
References about tent spiders also make note of parasitic "flesh flies" so the species interaction appears to be an established phenomenon.