Don't know if they still do but at one time QANTAS did not carry alternate fuel and would nominate an en route diversion, doing various calculations a couple of hours out, taking into account weather, runway and aids availability and fuel remaining, a decision would be made whether to continue to destination or go to the en route alternate. 99 times out of a hundred it was OK to continue to destination. Get held down for level, stronger than forecast headwinds, re-routing etc. would be the most likely cause of not making it to planned destination. Some operators call this method of despatch 're-despatch'.
Since the crew said it was a 'unique situation' none of the above may well apply.