The aim was to investigate the assortative sexual mixing by country of birth among heterosexuals in Australia. An analysis of 1060 heterosexual couples who attended the Melbourne Sexual Health Centre between 2011 and 2014 was conducted. Of the 1060 couples, 27% (n = 281) were both Australian-born men and women, and 42% (n = 445) were both overseas-born. Of the 171 couples with women aged ≤21 years, 41% (n = 70) were both born in Australia and 33% (n = 56) were both born overseas. A strong assortative mixing pattern by country of birth was observed among all 1060 couples (r = 0.361; 95% CI: 0.320-0.403), and among 171 couples with women aged ≤21 years (r = 0.481; 95% CI: 0.379-0.584).