Drinking under a bridge doesn't have the greatest connotation, but this bar makes that sentence completely acceptable. In fact, it's an awesome place to just chill with some mates. It can get a little crowded since it is just a little island under the bridge, but I just think it makes it cozy :)
This groovy little place has a really relaxed atmosphere and is great for some instagram shots. It's in a really accessible area as it is within walking distance from Flinders Street Station and you can just pop down for a drink or two while you explore Southbank.
It is worth noting that at times you may need to line up to get in as there isn't much space, and some of the drinks can be pretty expensive.