So now it turns out
1) he broke Serbian Covid rules after testing positive on 16 Dec, by mingling with the public on 17th and 18th.
2) There is great doubt as to whether he was really infected on 16th, since he was meant to provide the basis for exemption by 10 Dec, so to say it is "convenient" to then get Covid on 16th is an understatement. Without that "lucky" infection he could not have flown to Aus.
3) He lied on his application form by saying "no" to the question if he intends to travel in the 14 days before travelling to Aus, because he was in Belgrade on 25 December and in Spain on 31 December.
The Aus government has plenty of reasons to ban him, and to rescue their attempts to get their own citizens to follow the rules. Whether they do so at this point is purely political.
Looking all that up was 10 minutes of my life wasted.