I would be more confidant confirming this if I thought the food plant was a Sterculiaceae, but it looks rather like a Myrtaceae. I think it is therefore more likely the bag was made by some larvae of a Notodontidae or Oecophoridae species. A photo of the caterpillars would be nice, obtained by opening up the bag, but to determine the species, you would probably have to rear some in captivity to get a moth, which you could then take back and release at that site.
Have just added a photo of the tree, it was the very thin tree next to the big tree on the right. The bag was fairly high up and would not have been possible to reach. Sorry for not being able to help any more. Surprised you could see our listing as we put them on awhile ago and they are still not visible to us, so thanks for looking at it so quick.