One key consideration is when the cancellation notices were given. If it was a couple days before that would be one thing, if it was literally that I'm at the airport that would be another. Also, it is my understanding that for international flights, Qantas and other airlines are subject to MC99 and should they the cancel the flight, it is on them to put you on the next available flight be it with a partner like Japan Airlines or even a competing airline like United. They would also need to cover things like meals and hotel accommodation should they cancel.
In those instances, I would think that Qantas would find you whatever non-stop flight is available next and book you onto that. If it's with a competitor so be it. If they are unwilling to do that, simply walk up and buy the ticket, fly the segment and claim the funds back from QF. If they refuse take them to tribunal for not honouring the Australian Consumer Guarantee.
-RooFlyer88