What animals eat giant freshwater crayfish?

If they get to be adults, nothing except for humans! However, juveniles are preyed on by platypus, rakali (native water rats), large fish, such as the river blackfish and other crayfish -including adult giant freshwater crayfish.

Wild platypus in a creek in Tasmania Picture: Wikipedia. Klaus – Flickr.
Rakali. Hydromys chrysogaster, Dubbo NSW. Picture: Wikipedia. By ZooPro – Own work, CC BY-SA 3.0,
Tasmanian river blackfish (Gadopsis marmoratus). Picture: Tasmanian Inland Fisheries Service