Gastrointestinal parasites cost the Australian sheep industry AU$436 million annually. Early warning of impending worm risk may reduce this cost by providing producers sufficient time to implement control strategies. The provision of 90 day weather forecast data at a 6km grid resolution across Australia (Australian Bureau of Meteorology) has enabled the development of a mathematical model to predict the risk arising from nematode pasture infectivity for inclusion in the Sheep CRC's 'AskBill' application. A biophysical modelling approach was used to simulate the on-pasture lifecycle stages of 4 nematode species (Haemonchus contortus, Teladorsagia circumcincta, Trichostrongylus colubriformis and Trichostrongylus vitrinus). Mortality and development/migration rates of each lifecycle stage were described using modified β-distribution functions to account for the impact of temperature and water availability. The model was parameterised against point estimates from available literature and experimental data for the 4 species (H.contortus: R 2 = 0.88, n = 1409; T.circumcincta: R 2 = 0.56, n = 243; T.colubriformis: R 2 = 0.61, n = 355; T.vitrinus: R 2 = 0.66, n = 147). In the absence of a model predicting the quantity of eggs deposited, a probabilistic approach was used assuming 1 egg species-1 sheep-1 day-1 (thereby accounting for stocking rate). The impact of anthelmintic treatments were accounted for by assuming that no eggs were deposited for the duration of claimed efficacy. Risk was calculated by summing the proportion of infective larvae available for ingestion for each nematode species across each day of prior egg deposition and adjusting for herbage availability, species fecundity and productive impact.