Background This study examined trends in chlamydia, gonorrhoea and syphilis diagnosis, and chlamydia testing and positivity, among older women in Australia between 2000 and 2018. Methods: Using national notifiable disease data and Medicare data, diagnosis rates were calculated for each sexually transmissible infection (STI), as well as testing and positivity rates for chlamydia. Data were compared between two older groups (55–64 and 65–74 years) and two younger groups (15–24 and 25–34 years). Poisson regression examined trends for 2000–18 and 2014–18 separately. Results: Rates for all STIs increased across all age groups and were highest in the two youngest age groups. From 2014 to 2018, chlamydia rates increased the most among those aged 55–64 years [incidence rate ratio (IRR) = 1.06; 95%CI: 1.02–1.10] and declined in those aged 15–24 years (IRR = 0.99; 95%CI: 0.99–0.99). Gonorrhoea rates increased the most among those aged 65–74 years (IRR = 1.47; 95%CI: 1.23–1.77) and least in those aged 15–24 years (IRR = 1.12; 95%CI: 1.10–1.13). Syphilis rates increased the most among those aged 55–64 years (IRR = 1.58; 95%CI: 1.25–1.99) and least in those aged 15–24 years (IRR = 1.29; 95%CI: 1.23–1.35). Chlamydia test positivity declined among younger women but remained stable in older women. Conclusions: In general, STIs are increasing among older women in Australia at a faster rate than among younger women. Although the greatest burden is among younger women, STIs need to be considered and monitored among older women.