As a WP you can select any seat on the aircraft, however I would book two one way flights.

Recently I have found QFLink return reservations don't let you check in for the second sector. QF Premium said it's a known glitch and there's nothing they can do.

You won't be able to remain on the aircraft. You will have to disembark, clear security and go though the gate to the airport.

I have calculated this is also easily done at other QFLink ports like PQQ.