To (1) identify population-level classes of polysubstance use among young Australians between 2004 and 2016, (2) test if these classes changed over the same period, in terms of class prevalence and probabilities of substance use within each class, and (3) identify demographic and health-related correlates of polysubstance use.Repeated cross-sectional nationally representative household surveys.All Australian states/territories.Young adult samples (Aged 18-30; 58% females) from the National Drug Strategy Household Surveys (N = 20,350) MEASUREMENTS: Outcomes: Extent of past year use of 10 licit (e.g. alcohol) and illicit substances (e.g. cannabis) were used to derive polysubstance use classes. Correlates: Gender, age, psychological distress, general health, language background, personal income, education level, remoteness of residence, and socio-economic index for area of residence.Three polysubstance use classes were consistently identified between 2004 and 2016 (SSABIC: 188349): Minimal users (MU, ~60%), Mainly tobacco, alcohol and cannabis users (TAC, ~30%) and Extended range polysubstance users (POLY, ~10%). There were substantial changes in use of different substances within each class over the study period. For example, smoking decreased in all classes (p < .05) while harmful alcohol use only decreased in the first two classes (p < .05). Factors associated with TAC and POLY were similar over the study period. These included: being male and having an English-speaking background, a high level of psychological distress, suboptimal health and high personal income. Living in an affluent area was associated with reduced likelihood of being TAC but an increased likelihood of being POLY.At the population level among young Australians between 2004 and 2016, six in ten did not engage in polysubstance use; four in ten used a limited range of substances (mainly alcohol, tobacco, and cannabis), and one in ten used an extended range of substances. Over time, the types of substance within the extended polysubstance use class changed substantially.