The COVID-19 pandemic has increased food insecurity worldwide, yet there has been limited assessment of shifts in the cost and affordability of healthy, equitable and sustainable diets. This study explores the impact of the COVID-19 pandemic and income supplements provided by the Australian government on diet cost and affordability for low-income households in an Australian urban area. The Healthy Diets ASAP method protocol was applied to assess the cost and cost differential of current and recommended diets before (in 2019) and during the COVID-19 pandemic (late 2020) for households with a minimum-wage and welfare-only disposable household income, by area of socioeconomic disadvantage, in Greater Brisbane, Queensland, Australia. Data were collected between August and October, 2020, from 78 food outlets and compared with data collected in the same locations between May and October, 2019, in an earlier study. The price of most healthy food groups increased significantly during the pandemicâ€”with the exception of vegetables and legumes, which decreased. Conversely, the price of discretionary foods and drinks did not increase during the pandemic. The cost of the current and recommended diets significantly increased throughout this period, but the latter continued to be less expensive than the former. Due to income supplements provided between May and September 2020, the affordability of the recommended diet improved greatly, by 27% and 42%, for households with minimum-wage and welfare-only disposable household income, respectively. This improvement in the affordability of the recommended diet highlights the need to permanently increase welfare support for low-income families to ensure food security.