We used to "yabbie" all the time from our dams.
A bit of rabbit on a piece of baling twine, thrown into the dam for 20s, pull on the baling twine, and grab him by the tale (you have to be careful, a large one will remove your fingers).

They are quite tasty, but not all are big, the largest one I have caught was around 10inches in length. I don't know why/how they get so large.
Thier claws are the best eating.