the options for taronga have been discussed many a time before...
their option would be....
*transfer one or more of the adult females to another institution; Perth, Auckland or Australia Zoo as occurs quite routinely across zoos in the Northern Hemisphere, although this risks breaking up what is obviously a happy herd.
*expand the existing enclosure; doubling the space for the elephants by linking it up with the area currently occupied by fennec foxes, meerkats, pygmy hippos, dhole and asian birds. this is quite a deep shelf of land and very long. a Perth'esque style of enclosure could easily be adapted to this area.
*if the first two calves are male then by the time they reach 8 years old or so they could be moved down to the new bull facility where at least one could live permanently.
its highly unlikely any animals would be sent to dubbo whilst ever it still holds Africans due to the danger of transferring herpes virus and creating carriers in the breeding population. but with 2 years to plan im sure the zoo will come up with a solution.