Background <p>Nurse-initiated care improves timely treatment access, but adoption varies. The Ministry of Health introduced 73 Emergency Care Assessment and Treatment (ECAT) protocols for public emergency departments (EDs) to standardise nurse-initiated care. A behaviour change strategy was used to implement ECAT in 29 EDs as part of a trial, using implementation strategies like education, clinical champions, videos, and audit and feedback. The aim of this study was to evaluate the implementation of the ECAT protocol and the strategies used for reach, effectiveness, adoption, quality, and maintenance (REAIM).</p> Methods <p>This multi-method implementation evaluation used the RE-AIM framework. Methods comprised: i) emergency nurse surveys (Intervention and Control groups); ii) scoring by site implementation nurses of fidelity, adaptation and effectiveness of implementation strategies out of 5; iii) audits determining appropriateness of ECAT protocol use at 6–12 weeks and iv) implementation tracking logs during all study periods. Descriptive statistics were used for quantitative data, and content analysis for qualitative data (surveys/logs).</p> Results <p>Data from surveys (<i>n</i> = 787), audits (1375 audits, 509 nurses), fidelity scores (<i>n</i> = 11), and implementation tracking logs (&gt;600 entries) were used to evaluate implementation of ECAT protocols in the 29 EDs. Emergency nurses reported high use of ECAT protocols in their daily practice (median (IQR) 9.0 (8.0, 10.0), and audits demonstrated 92.9% of eligible nurses used at least one ECAT protocol in their shift (509/548). High effectiveness was demonstrated through 93.7% selection of the fully appropriate ECAT protocol (1288/1375). Implementation strategy fidelity was high (4.6/5) and effective (3.9/5), particularly the Education program (885 nurses, 97 courses), Audit and Feedback (1375 audits, 509 nurses), stakeholder engagement (&gt;450 meetings) and electronic medical record update (all sites). The strategy with the lowest fidelity and effectiveness was Clinical Champions. Maintenance was demonstrated by training, use and monitoring embedded in practice at 12 months.</p> Conclusion <p>Behaviour-change informed implementation strategies successfully enabled large-scale successful adoption of nurse-initiated care at 12 months. Relying on clinical champions already operating at full capacity in an overcrowded ED to assist with implementation was impractical. Ongoing investment in facilitation, education, and adaptive support structures is essential to maintain gains and realise the full potential of nurse-initiated emergency care.</p> Trial Registration <p>Australian and New Zealand Clinical Trial, ACTRN12622001480774p. Registered prospectively on 27 October 2022. Link: <a href="https://www.anzctr.org.au/Trial/Registration/TrialReview.aspx?id=384899%26isClinicalTrial=False">https://www.anzctr.org.au/Trial/Registration/TrialReview.aspx?id=384899%26isClinicalTrial=False</a></p>