Hi
the wetland has a problem at the Moresby River mouth where it meets Mourilyan Harbour.
https://tools.geofabrik.de/osmi/?view=areas&lon=146.11474&lat=-17.64157&zoom=13
The line across the mouth invalidates the multipolygon as the last water polygon of the Moresby River joins the wetland outer which is disallowed.
The way to fix this is to removed that line from the wetland multipolygon and instead use the bank of the waterways as part of the wetland boundary.
To do this you will need to make each of the waterway polygons into multipolygons and split them so that you have a separate line to use for the wetland following around the boundary of the waterways.
This problem reoccurs at the waterway mouths at the western side of the harbour and at the wooded area in that vicinity.
Let me know if this is too complex for you to correct.