OBJECTIVE: Mental health problems are an international public health issue affecting a substantial proportion of youth. This study aimed to identify groups of young children exhibiting distinct internalising and externalising symptom trajectories across early childhood compared to existing norms. Linear regression then identified child, parenting and family predictors from infancy in the development of internalising and externalising behaviours at age 5 years. METHOD: This consisted of a follow-up of 5-year-old children from a longitudinal, population-based study with earlier surveys having been completed by primary caregivers when the children were aged 7, 12, 18, 24 and 36 months. Five hundred 5-year-olds (68% retention) were included from 733 children recruited at age 6-7 months from routine well-child appointments across six socio-economically diverse government areas in Victoria, Australia. Mothers then completed a further questionnaire when their children reached 5 years of age, repeating the instruments included in previous waves. The primary outcomes were the Child Behavior Checklist (CBCL), 1.5-5 internalising and externalising symptoms (T-scores in relation to norms) and behaviours (raw scores). RESULTS: Across early childhood, three distinct profiles for each of the internalising and externalising symptoms (T-scores) were identified and compared to CBCL norms. Around 20% of this Australian child population exhibited consistently elevated symptoms for each problem. Regarding aetiology, longitudinally the strongest predictors of internalising behaviours at 5 years of age were harsh discipline, maternal stress, having no older siblings, single parenthood and maternal substance misuse. The strongest predictors of externalising behaviours at 5 years of age were male sex, harsh discipline and maternal stress. The predictors explained 22% of the variation in internalising behaviours and 24% of the variation in externalising behaviours at 5 years. CONCLUSIONS: Starting as early as the toddler period, effective population approaches to preventing mental health problems are needed. Randomised trials of preventive interventions focused on improving parenting practices and reducing maternal stress are under way.