Introduction Approximately 1 in 13 Australian children have a neurodevelopmental disability. This project aims to assess the effectiveness and implementation of an online parenting support programme, Parenting Acceptance and Commitment Therapy (PACT) Online, for parents of children with neurodevelopmental disabilities for improving the parent–child relationship and parent and child outcomes. Methods and analysis This hybrid type 1 randomised controlled trial will focus on evaluating intervention effectiveness and understanding the context for implementation. The primary outcome is observed emotional availability within parent–child interactions assessed at postintervention (12 weeks postbaseline) with additional measurement at follow-up (6 months postbaseline). Secondary outcomes include (1) parent-reported emotional availability, (2) parental mindfulness, (3) parent mental health, (4) psychological flexibility, (5) adjustment to child’s disability, (6) health behaviour and (7) regulatory abilities as well as child outcomes of (1) mental health, (2) adaptive behaviour and (3) regulatory abilities. Evaluation of implementation will include an economic evaluation of costs and consequences, and an implementation analysis grounded in the consolidated framework for implementation research with a focus on contextual factors influencing implementation. Ethics and dissemination Ethical approval has been obtained from the University of Queensland Human Research Ethics Committee (023/HE000040). Dissemination of study outcomes will occur through the appropriate scientific channels. Long-term implementation will be grounded within the implementation analysis and occur in partnership with the partner organisations and consumer engagement panel. This will include releasing the PACT Online intervention as a massive open online course on the edX platform if support for intervention effectiveness and implementation is found.