For those interested I have done some further research and discovered that in fact a beacon (as suggested earlier) was indeed erected around 1960.
XNATMAP home page provided all the info plus pictures.
This is the history:
THE DIVISION OF NATIONAL MAPPIMG’S GEODETIC SURVEY OF PAPUA NEW GUINEA – A PERSONAL PERSPECTIVE
Here is a pic:
HAJ in PNG 1959
And an album:
HAJ in PNG 1959
More albums can be found here:
XNATMAP - Photos
(Hope I haven't bored you
)
However the question still remains - when was the beacon replaced by a cross and by whom?