The prevalence of gestational diabetes mellitus (GDM) in South-Western Sydney is high, and this may be related to the diverse ethnic backgrounds of the residents. In 2013, the Australian Diabetes in Pregnancy Society (ADIPS) adopted new criteria for diagnosing GDM. We anticipate that the prevalence of GDM will increase, but it is not clear how that would be related to the woman’s age, pre-pregnant body mass index (BMI) and ethnic background. The aim of this study is to compare the prevalence of GDM at Liverpool Hospital using the old and new diagnostic criteria, with particular reference to the woman’s age, pre-pregnant BMI and ethnic background. We reviewed the results of all oral glucose tolerance tests (OGTTs) performed on pregnant women between February and December 2015 in our hospital, and determined whether the women would be diagnosed with GDM based on the old and new ADIPS criteria. Each woman’s age, BMI and country of birth were documented. There were 2140 OGTTs performed in 1725 pregnant women during that period. GDM was diagnosed in 14.8% (255/1725) of women under the old criteria, but this was increased to 29.6% (510/1725) under the new ADIPS criteria. The prevalence of GDM went up in all ethnic groups, but women from East/South-East Asia had the lowest increment (19.2 to 22.3%) while those from South Asia had the highest increment (22.0 to 44.4%). For women who were obese (BMI>30kg/m2), their risk of developing GDM was almost 50%. With maternal age, the risk of GDM increased with the higher age groups, but there was only a relatively small increment in women with age >35 years. Under the new ADIPS criteria, the prevalence of GDM could increase by 100% in some regions in Australia, depending on the demographic characteristics of women living in those regions.