The Fly flows mostly through the Western Province of Papua New Guinea and for a small stretch, it forms the international boundary with western New Guinea. This section protrudes slightly to the west of the 141°E longitude line. To compensate for this slight gain in territory for Papua New Guinea, the border south of the Fly River is slightly east of the 141°E longitude line. As part of this deal, Indonesia has the right to use the Fly River to its mouth for navigation.