Guys, I think you'll find that it's a fairly regular event in Darwin (and some other places too) to have one company sub-contract out a charter or part of a charter to another company.
ie; Company A picks up a charter for 4 aircraft to Timbuktu but only has 2 aircraft available. They then decide to call company B and ask if they can make up the shortfall with two aircraft.
As far as company B is concerned they are being hired to conduct a charter in their own right. It just so happens that they are being paid for that charter by another operator. The charter is conducted in their own aircraft and under their own AOC.
Hope I explained that well enough.... it's getting late and the brain isn't working too well tonight.