^^
Second that... What's your s.o.p's say? It is an approved document by casa. If it has scope to include duties and functions of a copilot position and you are fulfilling those duties and acting as a copilot. Then it don't matter if it's a single pilot approved plane. It only matters if under your company provisions as approved by casa, if it is a 2 crew operation and you have the right to be there and actually doing something.
I have operated on flights where the "contract" required 2 pilots (not 2 crew). Still single pilot ops as far as our sops went so guess what. I sat there logged nothing and twiddled my thumbs. 2 crew and 2 pilots are different things and unless your manuals approve it, then your just another passenger.