A 2014 SSO-ASTRO guideline on surgical margins aimed to reduce unnecessary reoperation after breast conserving surgery (BCS). We investigate whether publication of the guideline was associated with a reduction in reoperation in Western Australia (WA). In this retrospective, population-based cohort study, cases of newly-diagnosed breast cancer were identified from the WA Cancer Registry. Linkage to the Hospital Morbidity Data Collection identified index BCS for invasive cancer between January 2009 and June 2018 (N = 8059) and reoperation within 90 days. Pre-guideline (2009–2013) and post-guideline (2014–2018) reoperation proportions were compared, and temporal trends were estimated with generalised linear regression. The pre-guideline reoperation proportion was 25.8% compared with 21.7% post-guideline (difference −4.0% 95% CI —5.9, −2.2, p < 0.001, odds ratio OR 0.80 95% CI 0.72, 0.89, p < 0.001). Absolute reductions were similar for repeat BCS (16.3% versus 14.6%; difference −1.8% 95% CI —3.4, −0.2, p = 0.03) and conversion to mastectomy (9.4% versus 7.2%; difference −2.2% 95% CI —3.4, −1.0, p < 0.001). Over the study period, there was an annual absolute change in reoperation of −0.8% (95% CI —1.2, −0.5, p < 0.001). Accounting for this linear trend, the difference in reoperation between time periods was −0.5% (95% CI —4.3, 3.3; p = 0.81), reflecting a non-significant reduction in conversion to mastectomy. Comparisons of pre- versus post-guideline time periods in WA showed reductions in reoperation that were similar to international estimates; however, an annual decline in reoperation predated the guideline. Analyses that do not account for temporal trends are likely to overestimate changes in reoperation associated with the guideline. •The reoperation rate was lower after than before the SSO-ASTRO margins guideline.•However, an annual decline in reoperation preceded publication of the guideline.•Accounting for this trend, a smaller reduction was associated with the guideline.