In New Zealand and internationally, missed opportunities in the healthcare system to identify and reduce hazardous drinking among high-risk patients are very common. Following a brief alcohol misuse screen administered to over 6,000 injured patients, we will conduct a randomised controlled trial to investigate the effectiveness of an innovative mobile phone intervention. This will involve real-time motivational messages delivered periodically over 4 weeks using state-of-the-art technology and health psychology, social learning and communication theory. Our primary outcome of interest is reduction in alcohol misuse (AUDIT-C score) at 3 months. The effects will be reviewed at 6, 9 and 12 months. Through record linkage to hospital discharge, mortality and national accident insurance claims (a unique opportunity internationally), we will also investigate the impact on further injuries and other harms. This patient-centred intervention has the potential to be cost-effective, highly scalable and accessible to harder-to-reach communities, including youth and economically disadvantaged groups.