 The Australian government has used a semi-mechanistic modelling framework to track the spread of COVID-19 during the pandemic. The model uses behavioural data and case data when disease activity is present to estimate the transmissibility of SARS-CoV-2 from periods of high to low or zero case incidents. When the virus is active, the model recovers the effective reproduction number, which helps to monitor progress toward controlling large outbreaks. When the virus is not active, the model estimates the risk of reintroducing the virus into the population. This model has been used extensively in Australia to inform public health policy decisions and guide the country's response to the pandemic. This article was authored by Nick Golding, David J. Price, Gerard Ryan, and others.