Objective: Mental health service use by individuals without a diagnosed mental disorder is sometimes termed 'met un-need'. However, provision of services for this group may be necessary to provide appropriate assessment, referral and early intervention. This study quantified child and adolescent use of, and perceived need for, mental health services to inform population-level service planning. Methods: Young people in Australia's Young Minds Matter survey (n = 5837, 5-17 years), were categorised into four 'need' groups: (1) 12-month mental disorder diagnosis; (2) remitted for more than 12 months (or experiencing a condition not surveyed); (3) 12-month subthreshold mental health problem; and (4) no indication of need for help (i.e. did not meet the requirements of the first three categories). Service demand (use of, or perceived need for, a mental health service) and number of sessions received were estimated for each, separately for children (5-11 years) and adolescents (12-17 years). Results: Some 20.1% (95% CI: [18.6, 21.7]) of children and 32.3% (95% CI: [30.5, 34.2]) of adolescents expressed a demand for mental health services in the past year. Service demand decreased across the need groups. Perceived need without service use was higher among those with a 12-month subthreshold mental health problem (13.8/20.2%) than those who had experienced a mental health problem that had remitted for more than 12 months (or were experiencing a condition not surveyed) (9.3/12.6%). In addition, 23.6% of children and 24.6% of adolescents with a demand for mental health services were classified as experiencing no indication of need for help. Conclusions: This study quantified the number of children and adolescents in Australia who are likely to require mental health services. Findings suggest that not everyone in this group who has an expressed service demand meets diagnostic thresholds, but among those who do, service demand is higher.