Background Dual protection refers to the simultaneous prevention of sexually transmissible infection (STI) and unintended pregnancies. Optimal contraception and STI prevention strategies sometimes fail to align. Methods Using data from a large nationally representative population-based survey, we analysed the contraception and STI prevention behaviours at the last vaginal intercourse among 2420 heterosexually active women aged 16-34years who had participated in the Second Australian Study of Health and Relationships, 2012-13. Results At their last vaginal intercourse, most women (95%) used contraception and half (49%) used condoms, either as a sole multipurpose method or in conjunction with another type of contraception. Condom use was highest (72%) among women whose most recent partner was a casual or occasional partner, followed by women with a regular partner (59%) and women with a cohabiting regular partner (40%). One-third of the women (34%) used condoms as a sole method, and 14% used oral contraceptives together with a condom. Few women used implants or intrauterine devices (8%) and, among them, very few women also used condoms (<1%). Among the women who used a condom at their last vaginal intercourse, 49% reported both the correct use for STI prevention and consistent condom use during the previous 6months. Among women using condoms, correct and consistent use was also highest among women whose most recent partner was a casual or occasional partner (76%). Conclusions Although almost all women used contraception and half used dual protection, few benefited from the protective effects of using condoms together with highly effective contraception.