Admittedly it was some years ago, but I have had no problems with getting TK to tag to final destination when on 2 separate tickets with TK, i.e. TK to TK at IST. I have requested it a few times (in EU and the US) and it was never denied.

Whether OZ will be prepared to tag the bags to final destination, I have no idea, i.e. your OZ to TK flight is an OZ issue rather than relating to TK.

I see no issues if only 1 person enters Turkey.