The impact of pandemic influenza on the New Zealand Expeditionary Force (NZEF) in 1918-19 has never been studied using modern epidemiological methods. Therefore we analysed mortality and descriptive data from various sources for these military personnel. An estimated 930 NZEF personnel deaths from pandemic influenza occurred in 1918-19, making it the main cause of disease deaths, and representing 5.1% of all NZEF deaths from World War One (WW1). The epidemic curve was much more drawn out in the Northern Hemisphere compared with the Southern Hemisphere. Mortality rates varied markedly by setting (e.g. in military camps, by country and by hemisphere). Significantly higher mortality rates were found amongst NZEF personnel: aged 30-34 years, those of Maori ethnicity, those with a rural background, and those who left New Zealand for Europe in 1918. In conclusion, this work documents the heavy mortality burden from pandemic influenza amongst this national military force and highlights the large variations in mortality rates through host and environmental factors.