i wouldn't risk going too loud in melb.. you'll get EPA'ed pretty quick. I got EPA'ed for a perfectly legal 89db exhaust (see the pic.. lol)
Like Gizmo73 is saying, there is a lot more than just the size, the amount of bends, the angle of the bend, extra resonators, or muffler can all effect it. There is no "standard" way to say how loud it is just by looking at it.
You can ask brands, and you might get some feedback.