Well it's fairly certain the flights aren't operating.
You could ask to change to MEL-BNE instead...as it stands available both those days...but there's a high chance the Qld border will still be closed.
That pretty much leaves you with the option of asking for one of their 'conditional credits'. But these must be used for travel before Aug 15...and if not used, not guaranteed to be honoured by any new post administration Virgin.