why did Melbourne Centre need to advise him of unknown VFR traffic and then need Melbourne Centre to advise him once we had passed each other. Both on the same QNH and my electric slave holding an accurate altitude since 2 hours before? Was this a near miss or a near hit?
This is going to make you laff. We pass traffic on anything with less than 1000' betwixt that we believe warrant it (laterally speaking). If the traffic is based on un-verified mode C we use 2000'!
More, when I identified myself to Melbourne Centre as the unknown traffic and offered to climb if he was so concerned, I was ignored.
If there is 500' indicated between you the ATC had absolutely no concern about the conflict, he was just swimming at Bondi (going through the motions). You announcing yourself probably just confused him. In Class C airspace 500' would probably have been a valid separation standard, but he still would have had to pass traffic info.
Relevance to ADS/B? ATC are more likely to know a callsign for the unknown(?) traffic, depending on the VFR pilot's preference, which could be useful for discussions on the CTAF, irritating en-route...