What then happens to pax coming in on the aircraft from MEL? If the flight has departed MEL Int, then to use the lounge, they would have to come in through immigration, go to the lounge, back out through immigration and reboard. Not worth the effort.
Of course, if the MEL-PER is actually a flight that has departed MEL Dom, then everyone just alights as a dom pax and goes through Immigration in PER. But if the lounge is landside (as it currently is) then you'll have to do the whole tiresome immigration thing after leaving the lounge. (Yes, yes, I know that's what happens in ADL, but QF is clearly talking about a much bigger operation in PER).