There are a number of books about WW1 in New Guinea available in PDF format in the public domain. Here are some of them.
Australia versus Germany: The story of the taking of German New Guinea by F.S. Burnell. A first-hand account of the action in and near Rabaul, by the Sydney Morning Herald's Special Commissioner. (8 Mb)
Our new possession (Late German New Guinea) by J. Lyng. Description of New Guinea before, during and shortly after WW1. (8.5 Mb)
The neglected war: The German South Pacific and the Influence of World War I by Hermann Joseph Hiery. A thoroughly researched study into the effects of WW1 on German colonies in the South Pacific, including New Guinea, published in 1995 by an academic from the University of Hawaii. (15 Mb)