Jonathan W.
Yelp
They may be pushing the definition of "sanctuary", though the enclosures are a lot larger than most zoos. Healesville itself is a beautiful area. If you're in town without a car, don't be deterred by the seemingly long transit directions. The fairly long train ride out of Melbourne isn't too exciting, but it's direct. The bus you take is virtually a shuttle as you travel about 30kms with only a handful of stops, and brings you right to the zoo (there's no walk as my google directions provided). And the drive is gorgeous! You're driving through the Yarra valley and it's an experience in and of itself.
From looking at the website, Healesville has more of the Australian animals you'd be looking for than the Melbourne zoo - namely: koalas, kangaroos, platypus, etc... You even have a chance to walk amongst the kangaroos and pet them. The kangaroos there are from Kangaroo island, which are smaller (and safer) than others since there are no natural predators there and they didn't need to evolve accordingly. Note that you are supposed to book ahead for this experience and it can fill out. I was fortunate enough to walk in the area obliviously and still have the experience, but you may not be so lucky.
The zoo is nicely laid out, and a good size for about a 4 hour visit. Nice gift shop too, and fairly reasonably priced.