If a suitably equipped aircraft departs with a valid TTF that has no requirements, and once airborne, if the TTF does go below the alternate minima, but remains above the relevant landing minima, then the alternate requirement should go out the window. Particularly for east coast flying with one hour sector times.
(I accept that this probably would not have changed the unique VA/QF outcome at Mildura given the lack of CAT 2/3 capability at ADL)