The study used data from Murdoch Children's Childhood to Adolescence Transition Study (CATS), which has previously found that one in three children aged 8 to 9 years are experiencing bullying at their primary school on a daily or weekly basis.

This study is the first of its kind in Australia, following more than 1,200 Victorian children from Grade 3 into high school. Few studies have examined bullying prevalence in primary school and its impact on mental health.

One in four children reported they had been verbally bullied, while one in seven children experienced physical bullying. Up to one in 10 reported experiencing a combination of both.

Young children who were frequently bullied reported higher levels of mental health difficulties compared with those children who were not bullied.

The results provide strong evidence highlighting the need to promote positive peer relationships and implement bullying policies from the earliest years of school.

In early primary school, children are more amenable to interventions for bullying than older students. This could reduce the typical acceleration of bullying that occurs around the mid-primary years in Australian schools. Co-lead author, Dr Lisa Mundy said, "Peer relationships are particularly important during primary school and so it's important to support peer relationships during these years."

Murdoch Children's Director Professor Kathryn North said, "1 in 3 children experience frequent bullying. Bullying has a profound impact on children and has a major impact on their mental health. As a children's research institute, we are proud that we can provide parents with information that will help them support their children's mental health in early primary school."

"The Commonwealth Government's welcome Million Minds Mental Health Mission provides an opportunity to prevent mental health problems in children before they become chronic. Bullying in primary school should be a priority target for prevention research", said senior author Professor George Patton.

Researchers say mental health practitioners can promote the implementation of evidence-based anti-bullying programs in primary schools, such as increased supervision during breaks and firm disciplinary action for bullying.