73qanda,
It is difficult to understand some of the things you are saying.
Qantas or Virgin Australia jet, ... must ensure that at all times while airborne, there is sufficient fuel in the tanks ...
At "all times"? Are you serious? Do these words ("at all times") appear in your ops manual or some CASA rule? Or is this just your interpretation?
If the forecast for your destination was good before you departed, and you didn't load any alternate/holding fuel and your destination is REMOTE, how on earth can you say that, for the latter part of the flight, you have fuel to go somewhere else? How is the "at all times" being met, in this case?
... you simply carry YSCB from the YMML runway when you choose your departure fuel. Easy.
Yeah, easy - when talking about YMAV, YMML and YSCB. How about if you're going somewhere REMOTE? What do you do then?
I am still legal as I can currently reach YSCB.
But for how long will you remain "legal"? Soon after you set course for YSCB, you will cease being "legal" will you not?
And why do you use the word "legal"? Does your Ops Manual stipulate that you must have alternate/holding fuel on board even when the requirement for same was unforeseen prior to departure?