To kill the yabbies (freshwater crayfish) place them in the freezer for about 30 min or in a bucket of icy brine (salty water) for about 15 min. You need this brine at the end of the cooking process as well to chill the hot yabbies, stopping the cooking process!
There are many ways to cook them but simple is best, IMO, just add 250gm of salt to a pot of boiling water then throw them in. Small ones boil for 1 min 40 sec large ones 3 min 10 sec (wait for the rolling boil). Remove and chill in brine straight away. Plenty of recipes on the net for other tastes. Split green ones and BBQ with your favorite marinade is good too. Check the DPI & Fisheries sites in other states for regulations etc. Potato peel or rockmelon is a hot bait. Opera house pots do a good job of trapping them.