Aims To explore the acceptability of a telephone- or a group-delivered diabetes prevention program for women with previous gestational diabetes and to compare the characteristics associated with program engagement. Methods Postpartum women participated in a lifestyle modification program delivered by telephone (n = 33) or group format (n = 284). Semi-structured interviews on barriers and enablers to program engagement (defined as completing ≥ 80% sessions) were conducted before (Group) and after (Group and Telephone) interventions. The Health Action Process Approach theory was used as the framework for inquiry. Psychological measures were compared between engagement subgroups before and after group-delivered intervention. Results In the telephone-delivered program 82% participants met the engagement criteria compared with 38% for the group-delivered program. Engaged participants (Group) had significantly higher risk perception, outcome expectancy, and activity self-efficacy at baseline (P < 0.05). There was a greater decrease in body weight (−1.45 ± 3.9 vs −0.26 ± 3.5, P = 0.024) and waist circumference (−3.56 ± 5.1 vs −1.24 ± 5.3, P = 0.002) for engaged vs non-engaged participants following group program completion. Conclusions Telephone delivery was associated with greater engagement in postpartum women. Engagement was associated with greater reduction in weight and waist circumference. Further studies are required to confirm the effectiveness of telephone-delivered program for diabetes prevention in postpartum women.