Dr Michael Carr-Gregg, a psychologist specialising in adolescents, said children and teenagers were more vulnerable to mental health problems during periods of change, such as moving to high school.

''Depression comes at transition moments and those transition moments include starting secondary school and moving from one year level to the next,'' he said.

Other potential crisis times included losing a grandparent, being involved in a car accident or parents divorcing. He said parents needed to be especially vigilant during these times.

Dr Carr-Gregg said more than 160,000 young Australians lived with depression, which could start early in a child's life. It sometimes went undiagnosed for up to 15 years.

Parents should monitor their child's behaviour and familiarise themselves with the common symptoms of mental illness.

''What you're looking for is a change in their normal behaviour. One of the psychological jargon terms is the pre-morbid behaviour. That means, 'Before the change, how was he? How was she?' and of course that's incredibly telling.''

Signs of depression included disturbed sleep, overeating or a loss of appetite, avoiding school or friends, and restlessness or aggression.

For more on this story, see today's Canberra Times.