Introduction Digital health programmes not only complement existing services, but have the potential to reach populations that existing services are not reaching. Many of these services require people to opt-in to receive them, which presents significant barriers to access. An alternative is to make low-risk digital services opt-out, ensuring appropriate members of the target audience are signed up for a service unless they select to not receive it. Aim This study aimed to investigate how changing enrolment in a low-risk digital health programme from opt-in to opt-out would impact on enrolment and dropout rates. Methods This study involved the retrospective analysis of registration data from txtpēpi, a maternal and child health text-message programme. System-recorded data from enrolments during a 12-month period were obtained. In the first 6 months, users had to opt-in to the service (Period 1), but in the following 6 months, an opt-out process was implemented (Period 2). Results There was a 77% increase in enrolments in Period 2 (n = 113) compared to Period 1 (n = 64) and no significant change in the proportion of enrolments of Māori between time periods (P = 0.508). There was no significant difference in withdrawal rates between time periods at either 2 weeks (5% vs 6%, P = 0.676) or 1 month (9% vs 9%, P = 0.907). Discussion This study has shown switching from an opt-in to an opt-out option resulted in an increase in enrolments in an mHealth programme, but had no impact on withdrawals. This indicates that employing opt-out enrolment for low-risk evidence-based interventions is acceptable and a potential way to make these services more accessible.