I'm not sure I follow your logic there, you won't find the Australian species found in Australian zoos in the Exotic Mammals thread; to see a list of the native mammals held in Australian zoos you need to read that thread (
http://www.zoochat.com/24/native-mammals-australian-zoos-355163/).
The point of zoos is to display animals to the public, and for Australian zoos that means the Australian public, who will want to see exotic species at zoos (as you do), which is why there are a lot of exhibits devoted to exotic mammals.
Exporting endangered Australian animals would contribute 0% to saving their species, while using up available resources for supporting endangered species from countries where conservation is less well-developed.