Harms from gambling are a major public health problem in NZ. People with addictions, including problem gambling, respond to seeing ‘cues’ that make them feel strong urges to take part in the addictive behaviour. Behaviour-change programmes on computers can help people with gambling urges. The same programmes, developed and delivered by mobile phones, may be a way to reach and help many more people. Our study aims to determine whether a programme delivered via an app can help people who are having their lives harmed by gambling. We will work with consumers, to develop a smartphone app, then ensure it is acceptable to them, and easy to use. We will follow this with a randomised controlled trial that tests the effectiveness of the app for reducing gambling harm and preventing relapse, compared with a simple information (“placebo”) app.