Flug,
from what i understand the brisbane base is pretty content and works reasonably well. In melbourne with the growth of A380 flying there is consideration that it could be "managed" out of sydney and save on a lot of the Long Haul management costs as well as the flexibility of having AKL and SYD crew doing the non A380 flying.
The Company is certainly looking at its bases at the moment and the closure of the Melbourne base would allow certain flexibilities for the company that have arisen due to the A380.
In the case of a closure they would also be able to have a CR in the Melbourne base without affecting the operation of other bases. Given that Melbourne has the lowest FAAA membership maybe the company thinks that the FAAA wont put up too much of a fight? Of course they would have to offer positions in sydney for some of the crew