EPIRB
You're on thin ice when you say that a ticket is a legal contract. It is just a receipt for monies paid. It is not a guarantee to carry you anywhere, as Southern handler points out, due to any number of reasons.
However, if you are accepted for boarding a different set of rules come in to play (these vary from airline to airline) but it does not stop any airline from transfering you to another carrier to take you to any port so long as you are taken to your destination - even by bus... like Avalon to Tullamarine. Or they can refund monies paid.