Here we go again...
With an Alternate forecast on the TAF a Qantas aircraft would have the alternate fuel or would divert at or before PNR/DPA.
This whole debate hinges on the timing of the alternate declaration and its usually only an issue when an alternate condition arises apparently 'out of thin air'. That the TTF conditions can apparently go from no reasonable chance of alternate conditions (usually fog), to visibility below the alternate minima in a matter of minutes to me indicates a failure of considered weather observation and forecasting, not a failure of the airline's fuel policy.
Sometimes I suspect forecasters rely too heavily on their computer models, rather than taking a walk outside to have a look or listening to the bloke in the tower who's telling him the fog is "just over there" as evidenced in one of the aforementioned ATSB reports.