Almost a quarter of young Australians are living with "probable serious mental illness", according to a study.
The number of people aged 15-19 in psychological distress is higher than five years ago, said the report by a charity and a mental health group.
It also showed girls and indigenous Australians are more likely to suffer serious mental illness.
The report recommended more investment in evidence-based online support tools and improving mental health education.
Key areas of concern for young people include coping with stress, school and study problems, and depression, according to the Mission Australia and Black Dog Institute findings.
"Adolescence comes with its own set of challenges for young people," said Mission Australia chief executive Catherine Yeomans.
"But we are talking about an alarming number of young people facing serious mental illness, often in silence and without accessing the help they need."
Black Dog Institute director Helen Christensen said: "These findings confirm that mental illness is one of the biggest challenges of the 21st Century, and one that has to be tackled by the community, health services and families."