Particularly used to indicate that the listener is aware of the speaker's ingenuousness, for example where the person to whom it's directed feigns innocence or naivety.
Some sources say it is a WWII Australian Army expression. As to why it arose: one suggestion is that the reason lies in cooked prawn being more palatable than raw prawn. However, anyone from a fishing background who has handled raw prawn flesh, will know that it is limp, wet and slippery; like the sort of people at whom this phrase is directed.
Employee: "Don't you bloody come the raw prawn with me. I know it's you who wanted me out. At least have the guts to tell me to my face".