Objective: To help women with breast cancer (BC) be sufficiently active, accurate assessment of physical activity (PA) is important. This study compared self-reported and objectively-assessed PA in women with stage II+ BC. Methods: Moderate-to-vigorous-intensity physical activity (MVPA) was assessed objectively using accelerometery (Actigraph® GT3X+) and using self-report (Active Australia Survey) and reported in weekly metabolic equivalent minutes (MET.min/wk). Associations, difference and agreement between methods were evaluated using Spearman's correlation, Wilcoxon signed-rank test and Bland-Altman analyses, respectively. Women were also categorised as being insufficiently (≤ 450 MET.min/wk) or sufficiently active (> 450 MET.min/wk) and compared using Cohen's Kappa. Results: Participants (n = 50) were 51 ± 9 years, with a mean body mass index of 29 ± 6 kg/m . Self-reported and objectively-assessed MVPA were moderately correlated (r = 0.46, P < 0.01). Median MVPA was similar across methods (self-reported: median = 1085.1 [Interquartile range, IQR = 1032.2] MET.min/wk; objectively-assessed: median = 888.4 [IQR = 1218.5] MET.min/wk; P = 0.74), with wide limits of agreement (±2588.6 MET.min/wk). Conclusion: Self-reported MVPA using the Active Australia survey is comparable with objective assessments. For most who self-reported PA levels consistent with national guidelines, their objectively-assessed PA confirmed this to be true. Self-report methods can be used to identify women who need encouragement to stay active versus those who need targeted advice.