One day recently I was booked on the last SYD-MEL flight. Just after arriving in the J lounge they made an announcement saying if anyone who is booked on the last flight with HLO would like to move to the earlier flight, just come to the desk. I'm not sure if this announcement was made in the J lounge only, but I'm guessing it probably was, in which case they were effectively limiting this generosity to people with Platinum status or above, or companions of people with Platinum status or above.
Anyway, I would estimate that at least 20 people took them up on the offer, including myself. Even after adding us all in at the last minute, there were still numerous empty seats on my new flight, so perhaps that was a factor? Or is this a frequent occurrence coming up to the last flight of the day?