The study conducted by the University of New South Wales(UNSW) surveyed 2-thousand-300 Australians between 15 and 29.
It found three-quarters of them, who had sexual partners in the past year, had sex without condoms at least once in that time.
More than 70 per cent of those young people had regular sexual partners, and around a quarter didn't use condoms with casual partners.
It also showed that young heterosexual men with many casual partners are the least likely to get checked for sexually-transmissible diseases.
Professor Carla Treloar, from UNSW's Centre for Social Research in Health, says the study didn't aim to judge young people for their sexual activity, but rather consider the associated health risks.
