Some sharks migrate into the freshwater habitat of Brisbane's harbour for some months of the year. I believe they are bullsharks. We also get dolphins.
Another reason why rainbows, RTB, Bala 'sharks' etc are called that is due to just a name. A 'shark' name sounds much more appealing then Bala Cyprinid. Also, many are named 'shark' due to thier high dorsal and torpeado shaped body. Along with 'sharky' fins.
My 3 Bala's act so very skittish, and my 4" Red Tail Black patrols the tank as if a Great White is hunting for prey.