Biodiversity monitoring is crucial to understanding the status, distribution and trends in threatened species. With an increasing species extinction crisis, better methods are needed to design biodiversity monitoring programs and prioritise investment when conservation resources are limited. In this report, we developed a decision-theoretic framework for prioritising investment in threatened species monitoring while explicitly accounting for extinction risk, surrogacy, statistical power and monitoring cost. We applied our framework to prioritise and estimate the total cost of monitoring 1828 threatened species in Australia. We estimated that monitoring all of Australia’s EPBC listed species so that we can detect small-to-moderate declines with high (80%) power, will cost AUD $179 – 307 million per year depending on the extent of cost sharing.