Invasive species policy could be better informed if we understood how much people value reductions in the risks posed by these organisms. This study investigates the public's willingness to pay (WTP) for additional measures to reduce the risk of invasion of the Australian mainland by the Asian tiger mosquito (ATM). The study contributes to the literature by applying a stated preference method to estimate the public's WTP to reduce the risk of an ATM invasion, expressed as a change in probability. It is the first ex ante invasive species analysis to test over two discrete invasion reduction probabilities based on management effort. Further, to overcome the challenges in valuing changes in probabilities, the study presented respondents with a well-defined discrete difference in the final probability, with one scenario reducing risk from 50 to 25% and the other from 50 to 5%. We find a significant difference in the mean WTP values between these two scenarios (A$67 vs. A$90). The overall conclusion is that estimated benefits of reducing the probability of an ATM incursion outweigh the costs.