Yes - it's relatively normal - consecutive cabotage (aka "eighth freedom") means carrying passengers or cargo between two places in another country having start

Yes - it's relatively normal - consecutive cabotage (aka "eighth freedom") means carrying passengers or cargo between two places in another country having started (or intending to end) the flight in your own country. So the MEL-ADL leg would have DOH-DEL-ADL would be an example of that for a Qatari flag carrier. Cabotage as a whole is pretty unusual outside of integrated aviation markets like Europe. I would be very surprised if Qatar and Australia had an agreement that permitted it. In that absence of such an agreement, it would probably be illegal for QR to sell those tickets.