I was on QF456 on 30/3, which is the 1715 flight (I believe QF450 was 16:30?). When we arrived at the terminal at about 16:30 they were announcing that they're waiting for engineers, then they were calling for any people with connections. From the previous posts I see they eventually gave up on fixing the plane and got everyone onto another flight
I figure a fair few of them would have ended up in spare seats on QF456.
I guess we got lucky that we didn't book QF450 months ago when we were planning our trip!