APNG is an international operator and will have appropriate liability insurance as a condition of its Australian FAOC. Whether or not the PNG CAA provided proper regulatory oversight or adequate investigation, it was APNG's crash, not the PNG Government's. The Government may yet have to answer to ICAO on this and a number of other issues. But not the point here.
Because Australian citizens were involved, a case for compensation should be brought to court at an international level.
While it is not usual for investigations to apportion blame - but to find the cause - litigation lawyers don't operate under any such limitations. Once blame is established, if land has been polluted, the landowners may also be able to seek a settlement at a higher level or for a higher value than what is available locally. It probably depends on their ability to engage a suitable headkicking legal team. How is it the responsibility of the Australian taxpayer, via any special aid, unless it becomes some kind of urgent natural or environmental disaster? Unlikely that a passenger-carrying Twin Otter would be carrying enough toxins to do that. And if it was, that would open up another can of worms via the DG regulations.
Whatever, the insurers will be in their usual huddle to try and weasel out at minimum cost.