Yabbies? That's a new one on me! Crayfish, crawfish, crawdads... all the same. They'll eat whatever they can catch, be it animal or vegetable, dead or alive. If the fish are larger than the yabbies (I like that word!) then there probably won't be a problem, except maybe a torn fin now and then. Crayfish get a bad reputation as fish killers, when often the fish has actually succumbed to other problems, and the little pincher is just taking advantage of it.
John
P.s. When you collect your yabbies, keep an eye out for salties in the raupo!