I just did some poking around, and have come across two proposed dates:
1. Late 2012
2. Summer 2013/14
Now, maybe I'm just being hopeful (because I so really want a decent-sized water park in Sydney), but from what I can see, the Summer 2013 date seems to be from older news stories and the like. The 2012 one was on some stuff from the last few months (sorry, I was only having a quick look around, I don't have webpage sources at the moment), and yes, on the official (albeit not-very-updated) website too!
Just putting it out there...