I know, luck was on my side at the Sydney Aquarium and I got several very good Platypus shots, while I got none at Taronga, Melbourne and Healesville. It is for a reason that I think Sydney Aquarium's exhibit is the best; for visitors that is, can't comment for the platypus
With all due respect for Taronga, which I like very much; if I knew someone who only had a one day stop-over in Sydney and wanted to SEE a platypus, I would advise him to go to the Aquarium, not the Zoo. Even if Taronga's outdoor exhibit is good and naturalistic, you're probably just won't see anything in there unless you have hours to spare.
On a related note: do you know if the Zoo plans to continue keeping them in two locations: the outdoor exhibit in Creatures of the Wollemi and the Platypus and Wombat House?
I don't know, but I can't see why not. The Platypus House is the best place to see them, and there's no benefit in taking the animals out of the Wollemi.