This research used data from the Australian Child and Adolescent Survey of Mental Health and Wellbeing to estimate the lifetime and 12-month prevalence of suicidal behaviours in Australian young people aged 12–17 years, to describe their co-morbidity with mental illness and to describe the co-variation of these estimates with social and demographic variables. In any 12-month period, about 2.4% or 41,400 young people would have made a suicide attempt. About 7.5% of 12- to 17-year-olds report having suicidal ideation, 5.2% making a plan, and less than 1% (0.6%) receive medical treatment for an attempt. The presence of a mental disorder shows the largest significant association with lifetime and 12-month suicidal behaviour, along with age, gender, sole parent family status and poor family functioning. Of young people with a major depressive disorder, 19.7% reported making a suicide attempt within the previous 12 months. There are also significant elevations in the proportions of young people reporting suicidal behaviour who have anxiety and conduct disorders. Mental disorders should be a leading intervention point for suicide prevention both in the primary health sector and in the mental health sector specifically. The associations examined here also suggest that efforts to assist sole parent and/or dysfunctional families would be worthy areas in which to target these efforts.