Background: Prevalences of sexually transmissible infections (STI), unsafe sex and abortions are increasing in Australia and people aged 16 to 29 are particularly at risk. We conducted a survey of behaviour, knowledge and perceptions of STI risk among young people attending a longstanding annual music festival called the Big Day Out. Methods: A structured questionnaire was administered to a cross-sectional sample of people aged 16 to 29 years attending a music festival (Big Day Out). Results: Completed questionnaires were collected from 939 participants (507 females, 432 males) whose median age was 20 years. Of the participants, 751 (80%) had ever had vaginal or anal sex. In the previous year, 48% had multiple partners and in the past 3 months 66% had a new partner. Of these, 224 (39%; 30% of those who had ever had sex) did not use condoms all or most of the time and were classified as being at risk of STI; however, only 24% of those so classified perceived that they were at risk of an STI. In total, 43% of all sexually experienced participants had not used a condom because they reported being drunk or high at the time. STI knowledge was poor overall and male participants, those living in non-metropolitan regions, those under the age of 20 and those with less schooling scored relatively poorly. Conclusions: Our data suggest that young men and women who attend the Big Day Out are sexually active young adults with limited knowledge of STI and blood-borne viruses who regularly engage in behaviours that put them at risk of infection.