But in my honest opinion I reckon they will find an airline that already has the planes, and flies the route, and codeshare with them.
There are no airlines that operate the route directly and no aircraft currently that can fly the route. The planned range of the B777-8X is still a bit short for year round operation of JFK-SYD against the headwinds so the little mathematician who runs Qantas has asked Boeing if they can increase the range of that model to operate that route.