The Toronto zoo has a great Australasian exhibit. There are kangaroos, tree kangaroos, wallabies, echidnas, wombats, sugar gliders, lots of birds and reptiles like Komodo dragons, and a Great Barrier Reef area with sharks, seahorses, jellyfish, and more. I know it's not in the US but I thought it should still be mentioned.
Another small Australian exhibit that I have seen was at zoo Miami. It included koalas, tree kangaroos, emus, singing dogs, and crocodile monitors. Although it wasn't as big as Toronto's, it was still a decent exhibit area.
Other good Australian exhibits that I haven't been to are at the Cleveland zoo and the Fort Wayne zoo.