Purpose: Postoperative opioid prescribing frequently exceeds patient requirements, contributing to opioid-related harm and excess community supply. Following the international OPERAS study, Christchurch Hospital identified variability and oversupply in discharge prescribing. A guideline using opioid consumption in the 24 hours prior to discharge was introduced. We evaluated its impact on discharge prescribing and post-discharge opioid use. Methods: A prospective pre–post study was undertaken in a tertiary general surgery department. Adult patients undergoing eligible procedures were recruited using the OPERAS protocol. The prescribing guideline and consultant-led education were introduced. Discharge prescribing, post-discharge opioid use, pain severity, and patient-reported satisfaction at seven days were compared between pre-guideline (n=69) and post-guideline (n=77) cohorts. Rstudio was used to produce Pearson chi squares and non-parametric tests. Results: Baseline demographic and operative characteristics were similar between cohorts. Concordance with guideline recommendations was higher but not statistically significant in the post-guideline cohort (70% vs 58%, p = 0.126). The distribution and total quantities of opioids prescribed at discharge did not differ significantly between groups. Median post-discharge opioid consumption was low in both cohorts (0 vs 2 tablets, p=0.022). Opioid disposal information was infrequently provided (5% vs 8%, p=0.50). Patient-reported pain severity and satisfaction with pain management were similar between cohorts. Conclusion: Implementation of a consumption-based postoperative prescribing guideline improved alignment with a standardised framework but did not reduce opioid quantities prescribed at discharge. Guideline adherence alone may be insufficient where prescribing thresholds overestimate post-discharge analgesic needs. Broader stewardship approaches, including conservative caps, electronic prescribing supports, and integrated education, may be required to reduce excess opioid supply.