Hmm, there has been talk that it doesn't matter whether you fly DJ or QF, because you don't earn points/SC's anyway.
I've had both Plat Amex and Ultimate QF Amex for about 5 years, and despite what the policy says, that's never what has happened. All you have to do is when you speak to the Amex reservations person is asked them to put your FF number in so that the flight shows up online for you. They always say you won't get points (although last time they said to me that you are supposed to get SC's, but not points - not sure if that's true or not), however I've got points on every flight so far. The system doesn't seem to differentiate between a normal N/O class and the so called "free" flight. They seem book it like it's a normal flight booked through an agent. Maybe I've been lucky, but I do know someone else who's always earned points as well.
Anyway, from Oct 1 I'll be using the free hotel night. Seems better value to me to stay at the Hilton Sydney or somewhere like that, than to take a $49 each way flight with DJ. (And yes MEL-PER might be worth more than MEL-SYD on QF, but there's no way I'd sit in an uncomfortable Y seat for that long...)