601, Are you assuming they refuelled in Bulolo? Not a chance my contacts tell me there is no Avgas fuel in Bulolo. They would have to drive fuel from Lae in drums. Difficult for a white man to do that.
So, in order to speed up the enquiry we need to use the 'rubber hose', brass knuckles, thumb screws, to find out where they landed between Monto and Bulolo on their way to PNG, because round trip Monto to Bulolo to Monto is greater distance than Oaklands, California to Hilo, Hawaii, requiring massive ferry tanks. With 3 on board?? the flight there would not leave much space for anything unless they did a Cutmore job and took Jerry cans to top off with at Bulolo.
Lot of unanswered questions .. but with poor reporting in PNG, a very weak front line defence in Northern Australia, this indicates how ineffective surveillance is.
Well done to the various police forces involved, and well done to the PNG Police and to the local villagers in Bulolo.