"You're meant to be playing with dolls and scraping your knees falling over in the playground, not feeling sad all the time," she said. "Those feelings don't make sense to a little kid."

Now 19, the art student from Chippendale has been diagnosed with depression, anxiety, post-traumatic stress disorder and dissociation.

With new research showing almost one in four young Australians are likely to have a serious mental health problem, she believes more open discussion is vital.

"For a really long time everything had to be behind closed doors," said Bex, who asked for her surname to be withheld.

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"You couldn't talk about this stuff with your friends for fear of being labelled weird or strange and being bullied. That needs to change. Mental health problems affect so many young people. It needs to be something we talk about."

A joint report from Mission Australia and the Black Dog Institute has revealed high rates of likely mental illness in teenagers, with young women and Aboriginal and Torres Strait Islander people over-represented.

The Five Year Mental Health Youth Report, to be released on Wednesday, used a psychological distress measurement tool to assess the responses of thousands of people aged 15 to 19 from around the country.

It found the proportion of young people likely to have a serious mental illness rose from 18.7 per cent in 2012 to 22.8 per cent in 2016.

Over the five-year period, young women were twice as likely to be affected by mental health problems as males. In 2016, 28.6 per cent of female respondents met the criteria for likely mental illness, compared with 14.1 per cent of young men.

Older teens were at higher risk, with the survey finding 27.4 per cent of 18- to 19-year-olds showed signs of mental illness in 2016, compared with 20.8 per cent of 15-year-olds.

Mission Australia chief executive Catherine Yeomans said the main areas of concern for young people were coping with stress, school and study problems, body image and family conflict.

"There has been a significant increase in young people with a probable serious mental illness," she said. "We need to sit up and take notice so we can support them."

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."

The report makes a number of recommendations, including more investment in evidence-based online support tools and providing greater resources to schools to educate young people about mental health.

Guy McCulloch, a teacher at Burwood Girls' High School and co-ordinator of the Mindmatters mental health program, agreed schools could work to reduce the stigma associated with mental illness and better support students.

"There is a tidal wave of mental health issues in the schools," he said. "Teachers are spending more time helping students through mental health crises, particularly in years 11 and 12, where they can just crash and burn."