BACKGROUND: For mobile app based COVID-19 contact tracing to be fully effective, a large majority of the population needs to be using the app on an on-going basis. However, there is a paucity of studies of users (as opposite to potential adopters) of mobile contact tracing apps and of their experiences. New Zealand, a high-income country with western political culture, was successful in managing the COVID-19 pandemic, and its experience is valuable for informing policy responses in similar contexts. OBJECTIVE: This study pursues the following research questions: (1) How do the users experience the app in their everyday contexts? (2) What drives the use of the app? METHODS: Residents of New Zealand's Auckland region, which encompasses the country's largest city, were approached via Facebook, and 34 COVID Tracer app users were interviewed. Interview transcripts were analysed using thematic analysis. RESULTS: Interviews ranged in duration from 15 to 50 minutes. Participants ranged from those in late teens to those in early sixties. Even though about half of the participants identified as white New Zealanders of European origin, different ethnicities were represented, including New Zealanders of South Pacific, Indian, Mid-Eastern, South-American, and South-East Asian descent. Two of the participants identified as Maori (Indigenous New Zealanders). A broad range of careers was represented, from top-middle management to health support work and charity work. Likewise, educational backgrounds ranged broadly, from high school completion to Master's degrees. Two of the participants were unemployed, having recently lost their jobs because of the pandemic. The thematic analysis resulted in five major themes: perceived benefits, patterns of use, privacy, social influence, and need for collective action. Benefits of using the app to society in general were more salient to the participants than immediate health benefits to the individual. Use, however, depended on alert level, and tended to decline for many participants at low alert levels. Privacy considerations played little role in shaping adoption and use, even though the participants were highly aware of privacy discourse around the app. Participants were aware of the need for high levels of adoption and use of the app to control the pandemic. Attempts to encourage others to use the app were common, although not always successful. CONCLUSIONS: Appeals to civic responsibility are likely to drive the use of a mobile contact tracing app under the conditions of high threat. Under the likely scenario of COVID-19 remaining endemic and requiring on-going vigilance over the long term, other mechanisms promoting the use of mobile contact tracing apps may be needed, such as offering incentives. As privacy is not an important concern for many users, flexible privacy settings in mobile contact tracing apps allowing users to set their optimal levels of privacy may be appropriate.