Rainfed agriculture in Australia is vulnerable to an extremely variable agro-ecological environment, leading to high spatial and temporal variations in Australian grain production. National-scale grain yield estimation prior to harvest is an ongoing requirement of local farmers, stakeholders, banks, insurance companies, and logistics, as it reflects the significant influence of domestic and international grain market prices and demands. Therefore, early warning information on crop yield is very crucial for both farmers and decision makers. Climate is one factor affecting yield, crucial in some years but negligible in others when harvest conditions are controlled by other factors. Here we explore the relevance of climate forecast information from September onwards in setting the harvest yields. We develop a ‘harvest function’ that describes the utility of a climate forecast dependent on initial conditions such as existing soil moisture and crop condition.