Anxiety and depression are the most common mental health presentations in primary care and are currently treated by individual CBT-informed sessions by brief intervention counsellors. This may not be the most effective or efficient model of treatment. Group transdiagnostic treatment (TCBT) provides an efficient model of delivery of effective treatment that targets common factors and processes underlying anxiety and depressive symptoms. We hypothesise that this will allow for treatment of more people with the same clinician resource and be more effective than treatment currently offered. The trial is of 80 adults with anxiety and/or depression referred to brief intervention services in Christchurch. It will evaluate treatment efficacy and service delivery benefits (efficiency and cost-effectiveness) of group TCBT (which involves 10 participants over four, 2-hour sessions) compared with usual treatment (which is up to four 1-hour sessions of individual treatment by brief intervention clinicians).