This research study explored the usefulness of the Harmonised mHealth application (app) for whānau in supporting taitamariki to have healthy relationships. Harmonised is a Māori centred yet inclusive intervention co-developed with taitamariki to promote healthy relationships for taitamariki and reduce whānau violence. Whānau can be invited by taitamariki to join the app to access resources and provide advice if asked. The usefulness of the app for whānau was explored using a new approach, user experience co-inquiry, within a qualitative descriptive methodology. User experience co-inquiry is a clinician facilitated and user led usability inquiry that takes the form of a semi-structured interview. Six whānau, all mothers, were interviewed, three of whom identified as Māori. The results showed that Harmonised was viewed positively by whānau as a useful resource to support taitamariki to have healthy relationships. The circumstances, concerns and needs the mothers had for taitamariki framed the way in which they experienced the app. Thus, the usefulness was viewed in terms of facilitating communication and ensuring the safety of young people. Suggestions were made by the mothers for additional content and enhanced functionality of Harmonised to be considered in future releases. Latent analysis of the data addressed a gap in the literature, by eliciting a potential model for evaluating the usefulness of eHealth and mHealth interventions. This model posits that the needs and concerns of the user determine the way in which the usefulness and usability of the app would be experienced. Thus, a user centred approach to eHealth and mHealth usability is beneficial to support clinicians make recommendations to clients and to inform future on-line interventions.