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Background: We quantified the individual and joint contribution of contemporaneous causal behavioural exposures on the future burden of oesophageal and stomach cancers and their subtypes and assessed whether these burdens differ between population groups in Australia, as such estimates are currently lacking. Methods: We combined hazard ratios from seven pooled Australian cohorts (N = 367,058) linked to national cancer and death registries with exposure prevalence from the 2017–2018 National Health Survey to estimate Population Attributable Fractions (PAFs) with 95% confidence intervals (CIs), accounting for competing risk of death. Results: Current and past smoking explain 35.2% (95% CI = 11.7–52.4%), current alcohol consumption exceeding three drinks/day 15.7% (95% CI = 0.9–28.4%), and these exposures jointly 41.4% (95% CI = 19.8–57.3%) of oesophageal squamous cell carcinomas in Australia. Current and past smoking contribute 38.2% (95% CI = 9.4–57.9%), obesity 27.0% (95% CI = 0.6–46.4%), and these exposures jointly 54.4% (95% CI = 25.3–72.1%) of oesophageal adenocarcinomas. Overweight and obesity explain 36.1% (95% CI = 9.1–55.1%), current and past smoking 24.2% (95% CI = 4.2–40.0%), and these exposures jointly 51.2% (95% CI = 26.3–67.8%) of stomach cardia cancers. Several population groups had a significantly higher smoking-attributable oesophageal cancer burden, including men and those consuming excessive alcohol. Conclusions: Smoking is the leading preventable behavioural cause of oesophageal cancers and overweight/obesity of stomach cancers. [ABSTRACT FROM AUTHOR]