Gestational diabetes mellitus (GDM) is associated with adverse maternal and neonatal outcomes. In 2013, the Australasian Diabetes in Pregnancy Society endorsed new diagnostic and treatment guidelines for GDM in Australia. Eastern Health adopted the new guidelines in 2015. The aim of this study was to compare two annual cohorts of women and their neonates, before and after the implementation of the new guidelines. Sociodemographic and clinical characteristics, treatment burden and maternal and neonatal outcomes were compared. In additional, maternal characteristics associated with adverse maternal and neonatal outcomes were examined. A total of 1,335 pregnancies affected by GDM were studied. Sociodemographic characteristics were generally similar between the cohorts. There was no significant difference between the proportion of each cohort diagnosed with GDM with introduction of the new guidelines (13.9% in 2014 compared to 14.7% in 2015). There was a significant reduction in the number of patients with gestational diabetes treated with diet alone (65.42% compared to 44.73%), and a significant increase in the number of patients treated with insulin (32.09% compared to 53.1%). The adverse outcomes of gestational hypertension, neonatal hypoglycemia and neonatal jaundice were significantly reduced following introduction of the new guidelines. There was no difference in in rates of induction, caesarean section, or admission to special care nursery. The introduction of new diagnostic and treatment criteria have increased the number of patients requiring insulin. This has had minimal impact on obstetric and neonatal services, however workload for the diabetes and endocrinology department has been increased.