Gestational diabetes mellitus (GDM) is an important and increasing health problem. This study aims to investigate and explain the marked variation in reported GDM prevalence among Australian Indigenous women.We searched five databases to August 2013 for studies of GDM prevalence; two people independently assessed search results, extracted data, and appraised risk of bias. Meta-analysis was conducted, and between-study heterogeneity examined using subgroup analyses. Within-study findings were synthesized narratively.The pooled GDM prevalence from 23 of the 25 total studies (5.74%, 4.78-6.71) was similar to that reported in national studies, but heterogeneity was substantial (I(2) = 97%), making conclusions from between-study comparisons difficult. The greatest reductions in heterogeneity were seen within subgroups using localized diagnostic criteria (I(2) = 43%, 3 studies), universal screening (I(2) = 58%) and some jurisdictions, probably reflecting proxy measures of increased consistency in diagnostic and screening methods. Insufficient data were available to assess the effect of factors such as rurality, diagnostic criteria, study design and data sources on prevalence. Synthesis of within-study findings showed: higher age-adjusted prevalences of GDM in Indigenous versus non-Indigenous women; Indigenous women have greater increases in prevalence with maternal age; and non-Indigenous women appear to have a steeper increase in GDM prevalence over time. Prevalence increased almost fourfold in two studies following introduction of universal screening when compared with selective risk-based screening, although numbers were small.The published GDM prevalence among Indigenous women varies markedly, probably due to variation in diagnostic and screening practices.