Datal Seniority serves to regulate labour supply in favour of the employer. I know that this is not a popular perspective to hold, however, I must speak what I see as the truth.
If you abolished that then salaries in Australia would tank as you would be invaded by Britts, Kiwis, South Afrikans, and expat aussies who are working for foreign airlines. It would be an employer's dream! Just take a look at Jetstar and VOz.
At least seniority gives some value to living in Australia and protects the locals from cashed up expats who will undercut the local market just to get back to Oz.