Akro, it's Part 23. Raisebeck does have a Part 25 supplement though.
3100lbs (full mains and 400ltrs in the aux tanks) for a flight to King Island and back? That's a heck of a lot of fuel! With 5 on board and gear, that could well be pushing limits depending on the BEW of the aircraft. Most would struggle to load 500kg (pilot/pax/bags) with that amount of fuel on board.
As for the stated cause in the report, I'm speechless.