Beerman, yeah, you are absolutely right - the current planned return time to SYD is too late to turn the jet around and send it to Europe. There is one late departure, the QF21 to Tokyo (departs 2205 at present) but even that is too tight given that the aircraft would have to be towed across to the international terminal also.
So it's probably more a combination of arrival times from long-haul overseas constraining the departure time from SYD to PER. And I expect this is more a case of using 2 or 3 different airframes on certain sector combinations over a 48/72 hour period, not just 1 airframe coming in from o/s, doing a quick SYD/PER/SYD and then immediately heading o/s again. More likely 1 aircraft in from o/s in morning, do a SYD/PER return immediately then night stop SYD for maintenance, o/s again next morning.
I really don't know what they are planning (it's a long haul type and not my area). If it turns out to be a dedicated airframe doing SYD/PER/SYD each day, then yeah, it ought to be on the red-eye. But I would also question the rationale for dedicating such a big aircraft to a relatively short sector.