Of course it's a fuel dump. That'll be why the big jets have a pipe sticking out of the wing trailing edge
Fuelled for Trans-Pacific flight (presumably this flight stops over at Honolulu?) so above max landing weight for return to Sydney.
If you watch the following video from the first link it claims they lost an engine due to bird strike. Unclear how far into the flight they were but that's a long way to go on one engine. ETOPS probably made the decision for the crew.