Healesville Sanctuary was just part of the tour I'd booked with Gray Line. (They've got a lot of different tours that include a trip to Healesville Sanctuary.) It was my favorite part, however.
At Healesville, they ONLY present native Australian wildlife, so it's not quite like a zoo where they have other animals as well. There are free flight aviaries where you will get closer to rare birds (like the male lyrebird I saw) than you will most places, but they still really aren't on "display". You have to look to see them.
This is one of the things that friends who live in Melbourne suggested I go to see and I'd have to say they were right. It was a very enjoyable way to spend an afternoon.