JORN
As far as I can remember the JORN system is more broad based surveillance but can be focused in certain areas if required. So if there was intelligence that such a flight was happening then they could certainly track it across to PNG and back.
But from all the articles I’ve ever read about it, JORN doesn’t see everything, everywhere, all the time. Plus I think the state of the ionosphere can affect it as well (it bounces radio waves to get ‘over the horizon’).
Otherwise why would the occasional refugee boat sneak through? Or what about MH370, the coverage maps show JORN has the potential to see out that far.