The University of Otago (Wellington) study is believed to be the first in-depth examination of the non-fatal health impacts suffered by New Zealand military personnel as a result of the 1914-18 conflict.
Lead author Prof Nick Wilson said behind official figures for those wounded or sick in World War 1, there was an "enormous" amount of other suffering that was not officially reported.
"Much of this was severe enough to send these soldiers to hospital, or to result in their discharge from the military."
The study analysed the military files of 200 randomly selected World War 1 army veterans.
It found 94% had at least one new health condition diagnosed during their military service, and 89% were admitted to hospital at least once for a new condition.
He said the most common diagnoses were for infectious diseases, which were at more than double the rate of conflict-related injuries.
Respiratory conditions such as influenza, pneumonia and tuberculosis affected 33%, and 14% were diagnosed with sexually transmitted infections.
Diagnoses suggestive of post-traumatic stress disorder (PTSD) were present in 10%, and chemical warfare injuries were in 6%.
Study co-author Associate Prof George Thomson said the analysis suggested the true rate of illness and injury was more than twice what official figures suggested, and on average these soldiers were admitted to hospital 1.8 times for new conditions.
Added to this, 59% of personnel were deemed no longer fit for military service at some stage.
He said it had taken more than 100 years to establish the morbidity burden for New Zealand military personnel in terms of new conditions and hospital admissions from World War 1.
Prof Wilson said the results showed governments that sent military forces into combat needed to provide adequate protection from injury and other health risks, including proper protective equipment and good medical services.
