Objective: To estimate the prevalence of polypharmacy among Australians aged 70 years or more, 2006–2017. Design, setting and participants: Analysis of a random 10% sample of Pharmaceutical Benefits Scheme (PBS) data for people aged 70 or more who were dispensed PBS-listed medicines between 1 January 2006 and 31 December 2017. Main outcome measures: Prevalence of continuous polypharmacy (five or more unique medicines dispensed during both 1 April – 30 June and 1 October – 31 December in a calendar year) among older Australians, and the estimated number of people affected in 2017; changes in prevalence of continuous polypharmacy among older concessional beneficiaries, 2006–2017. Results: In 2017, 36.1% of older Australians were affected by continuous polypharmacy, or an estimated 935 240 people. Rates of polypharmacy were higher among women than men (36.6% v 35.4%) and were highest among those aged 80–84 years (43.9%) or 85–89 years (46.0%). The prevalence of polypharmacy among PBS concessional beneficiaries aged 70 or more increased by 9% during 2006–2017 (from 33.2% to 36.2%), but the number of people affected increased by 52% (from 543 950 to 828 950). Conclusions: The prevalence of polypharmacy among older Australians is relatively high, affecting almost one million older people, and the number is increasing as the population ages. Our estimates are probably low, as we could not take over-the-counter or complementary medicines or private prescriptions into account. Polypharmacy can be appropriate, but there is substantial evidence for its potential harm and the importance of rationalising unnecessary medicines, particularly in older people.