my exhaust guy (Hillside exhausts) in furntree gully sorted me out when it happened to me, basically he bolts up a stock (or quiet little pea shooter basically) exhaust go pass the noise (he cant do them unfortunately) test drive back to the workshop then he puts your normal exhaust back on for ya. Cost me $50 or 100 I think cant remember but made it pretty easy for me.
mine passed with a straight through 3" 'cat' too. (33GTR)