Gestational diabetes mellitus (GDM) during pregnancy is associated with health complications for both mother and infant, but patient numbers in the Waikato District Health Board region of New Zealand have not been well characterised. This study reviewed the full 2018 cohort of Waikato District Health Board hospital births (n = 4970) to report on GDM prevalence by ethnicity and age. The overall prevalence of GDM was 5.7% and is more likely to affect Asian, Pacific and Māori women as well as those of advanced maternal age.