Inclusion Criteria:

• Families on waiting lists and child and adolescent mental health services clinics with children aged 2−5.
• Scores in the clinical range on a standardised measure of child behaviour problems.
• At least one parent in each family must be literate, fluent English speakers to participate because the self-administered intervention will primarily consist of written instructions and information in English.

Exclusion Criteria:

• Clients whose children score below a clinical cut-off score on a standardised measure of child behaviour problems. This is to ensure that participants are appropriate for an intervention aiming to reduce behaviour problems.
• Non-English speakers or those who are unable to read cannot be included because reading English is required to complete the self-administered intervention.
• Children or parents with severe disabilities and children with a developmental disorder (e.g., autism) will be excluded because the version of the parent training programme that will be implemented is not designed for families with these types of problems.
• Children who live with a temporary caregiver will be excluded because the intervention is designed for full-time parents and follow-up will be after one year.
• Children or parents who are currently receiving treatment for psychological problems will be excluded because outcomes may be influenced by interventions not affiliated with this project.