The finding is from a Burnet Institute survey of more than 3500 people aged 16-29 who attended the Big Day Out between 2009 and 2011.

Researchers asked participants a range of questions about their sexual health knowledge and behaviour including their age when they first had sex.

Overall, the median age at which participants first had sex was 17.

But 29 per cent of the group reported first having sex at an early age, defined as younger than 16 years.

Participants aged 16 to 24 were more likely to report having sex early, compared to participants aged 25 to 29 years.

Researchers found that participants who had sex early were more likely to be male, live outside major cities and have no post-secondary school education.

They were more likely to have used illicit drugs and binged on alcohol more than weekly; and less likely to live at home with their parents than in other living arrangements.

Those who had sex early had a better knowledge of sexually transmitted infections, but were also more likely to report inconsistent use of condoms.

Authors of the study, published in the Journal of Sexual Health, said the findings reinforced the need to deliver sexual health information to young people in a timely way.

"Given the young age at first sex identified in this study, and given that the age of onset of puberty has declined in Australia, it is vital to deliver comprehensive sexual health education and inform young people about sexual health at a younger age, before sexual debut," they said.

Researchers said sexually transmitted infections such as chlamydia were becoming increasingly common among young people, and could cause serious and long-term health consequences such as infertility and pelvic inflammatory disease.

A La Trobe University survey released in May found 23 per cent of year 10 students were sexually active in 2013 compared to 27 per cent of year 10 students in 2008.

But Burnet researcher Alyce Vella said research indicated the age young people were first having sex had been steadily declining over the longer term.

She said data on young people collected through conventional means, such as telephone and school-based surveys, were "often biased towards lower risk individuals".

Recruiting young people at the Big Day Out was a novel approach that created a "relaxed environment" for them to complete the survey, Ms Vella said.

Researchers said exposure to sexual content could be part of the reason for a historical decrease in the age young people were first having sex.

"Sexualised content such as pornography has become normalised in today's society and provides a private and easily accessible way for young people to learn about sex and sexual norms," they said.

Ms Vella said the internet and social media also offered opportunities to provide sexual health information outside the classroom, with work underway to understand how best to deliver such information to young people.