The investigation into this will be a paperwork exercise of whether they should have carried x amount of fuel, was a diversion prudent, was the delay appropriate. At no point was safety really a factor as they landed with more than enough fuel, as the crew did the right thing and made sure they landed with enough. If they held until fuel went down to fumes, then it would be a safety event.
So this crew arrived over PH with 1600 kgs if the 40 mins fuel remaining is to be believed.Given 15mins extra holding would be max.600 kgs or if on descent less than 100 kgs.They must have been arriving PH with 2200kgs max. including any traffic holding from the flight planning stage.This is very thin on fuel for PH.This airport always needs to be considered a remote strip operation with unpredictable WX and the worst ATC system around, you ALWAYS need a plan B.So there is plenty to see here!!What is legal and what is safe,are two different things.
Maybe they were using to old plan for KG and keep going as contingency reserve requirements reduce allowing Perth.