Purpose Patient complaints provide a complementary lens on surgical safety, yet prior analyses have been small and specialty-specific. We aimed to characterise themes, perioperative processes, systems-factors, and outcomes in surgical complaints using a scalable, large language model (LLM)-assisted approach. Methodology We performed a national retrospective study of publicly available investigation reports from the New Zealand Health and Disability Commissioner. All reports published since 1998 were retrieved and screened for relevance across all surgical specialties. A fixed-parameter LLM workflow extracted demographics, clinical context, complications, breached patient rights, system factors, and outcomes. An inductive LLM-supported thematic synthesis generated complaint themes, followed by re-scoring of theme relevance across reports. Human validation supported the LLM-assisted process (Cohen’s kappa >0.85). Results Of 1,827 reports screened, 650 involved surgical care. Postoperative complications were frequent (84.2%), commonly accompanied by permanent disability (33.2%) or death (24.2%). Delays in recognition of deterioration (76.1%), escalation (58.3%), and definitive management (49.5%) of complications were common and associated with higher mortality. Prominent themes concerned postoperative clinical management/monitoring, communication/informed consent, and professional conduct/competence. Technical/procedural errors and medication errors were comparatively less common. Breaches most often related to reasonable care and skill, and to informed consent, with marked variation across specialties. Conclusion Complaints about surgical care predominantly reflect postoperative monitoring, escalation, and communication rather than intraoperative technical errors. System priorities should include robust deterioration recognition and response, reliable handover and escalation pathways, and strengthened consent and communication processes.