That's got me interested now!
From looking on Flight Radar 24, it looks as if one of the A380s must have had some kind of an issue not serious enough to stop it flying but requiring attention at the earliest opportunity and needed to be rotated back to Sydney for attention as it looks like the aircraft was switched in Singapore.
The screenshots below show the flights in question and the last movements of both the aircraft involved. It's not completely clear because there are apparant duplicate moves showing the airfraft to be in two places at once, but it does look as if the aircraft for QF1 and QF2 were swapped over at Singapore. One of them also seems to have been swapped completely for a third aircraft!
My head is hurting trying to work this out... is there anyone on here who has experience of diagram planning who could look at the screenshots below and try to put it more clearly?