Now that we've all agreed that Austrailia is in here because the community requested it as opposed to their historical contributions, and that depending on your point of view, this is either ok or the worst thing Firaxis could possibly do...
Lets discuss gameplay! Austrailia's abilities give it some interesting motivations other civs don't have. First of all, a city on a costal tile gives +8 housing, which is huge, and probably the best part of their ability. Yes, its fewer tiles to work, but it means you can delay getting housing for much longer than other civs. How can Australia abuse this advantage to get ahead in other ways? I'm torn. On one hand, you don't need to build as many farms for housing purposes, which means you can build your districts earlier and get bonuses from them sooner, without sacrificing growth or hitting the housing limit! On the other hand, you could also build farms to grow your cities huge early without being restricted by growth. I'm not sure what the best balance will be.
I feel like the harbor district is surprisingly not going to be a huge priority for Australia. You'll eventually get the adjacency bonus for them when you put it next to your commercial hubs, but the rest of the bonuses a harbor provides are pretty minimal for them. The housing provided by harbor buildings aren't going to be very important, and you can already build ships from the start since you'll want cities by the coast. Eventually, you'll probably want them for the trade route, but that's about it. Commercial districts, on the other hand, will be great.
The outback station seems pretty good, and is definitely one of the best unique tiles, but it will be interesting to see how many places you can spam them, considering the relatively late place they come in the tech tree, and the fact that your cities will be primarily costal. It probably won't be a game changer.
The digger will be almost useless unless its bonuses are huge. It's both an infantry unit and its late in the game, which is not a good combination for a UU.
Citadel of Civilization sounds situationally great, thats a pretty huge production bonus. In single player it means you have a pretty big incentive to be the world police while annoying everyone. If Austrailia becomes a top civ in MP, it will be because everyone is too afraid to go to war with them.
No unique district hurts them in terms of civ tiering, so I'd say this is probably a middle of the road civ, but it will be interesting to see how these bonuses work out. I could be wrong.