Seniority in Australia is also what keeps many very qualified and experienced pilots from around the world (ie. Australian expats) who could bring a lot to the system, from coming back to Australia into suitable roles (ie. Commands and Check and Training roles), all because if that very experienced pilot wants to come back to Australia, they are demoted to a First Officer (or Second Officer in Qantas' case!).
Seniority works to a point, but it's a system that's still not in the best interests of the company in terms of recruiting the best people for the job into the most appropriate roles.