Strict seniority in any company EBA is Not a guarantee in this day of age. ,!!
I don't work for qantas but feel very sorry for the stress you guys/girls must be going through. (I have gone through it myself both in aviation and prior).
I will say however, no agreement / EBA these days of guaranteed last on - first off / strict seniority etc, means much these days. Companies can easily go to FWA (particularly when in hardship and losing hundreds of millions) and if they can show those agreements cost more than than the proposed redundancies (and hence even more company hardship and even further redundancies), can argue and will probably win, exclusion from those previous agreements.
I kid you not, do not take those clauses for granted and think your safe. Work together to do anything for a deal that is remotely acceptable to the most, otherwise the company will win in FWA. Good luck!