According to their website, December 2010. But considering they hadn't started construction when i visited a couple of weeks ago, i'm doubtful it will be ready by December.
The new exhibit will be next to the current giraffe/zebra enclosure where the bongo are now. You may be able to see the giraffe/zebra enclosure on the right of the plans above.
I've just seen the Zoos Victoria annual report and it states that the exhibit should be ready by early 2011.
I can only assume the bongo will be moved to another zoo, maybe WORZ. It will be interesting to see what happens to the corner of the zoo the current baboon exhibit is in.