Gastrointestinal parasites cost the Australian sheep industry AU$436 million annually. Early warning of impending worm risk may reduce this cost by providing producers with sufficient time to implement control strategies. A biophysical model was developed to simulate the on-pasture lifecycle stages of the four predominant nematode species in Australia (Haemonchus contortus, Teladorsagia circumcincta, Trichostrongylus colubriformis and Trichostrongylus vitrinus). The influence of climatic variables (temperature and water availability) on the survival, development and migration of each lifecycle stage was incorporated and parameterised to available point estimates (H. contortus: R² = 0.88, n = 1409; T. circumcincta: R² = 0.56, n = 243; T. colubriformis: R² = 0.61, n = 355; T. vitrinus: R² = 0.66, n = 147). Constant fecundities (eggs/worm.day) provided the daily quantity of eggs deposited per sheep (H. contortus = 3275; T. circumcincta = 140; T. colubriformis = 300; T. vitrinus = 300). Farm management practices were considered via the specification of stocking rates (sheep/ha), and the administration of anthelmintic treatments (reducing egg deposition by a defined efficacy and duration for each nematode species). Pasture infectivity per nematode species was calculated as the quotient of larvae on herbage and herbage availability (t/ha). Risk was calculated as the product of pasture infectivity and the potential productive impact of each nematode species (H. contortus = 3.9%; T. circumcincta = 9.22%; T. colubriformis = 9.31%; T. vitrinus = 9.31%), and then summed across nematode species. This predictive model has been incorporated into the Sheep CRC's 'ASKBILL' application (www.askbill.com.au, verified 13 April 2018), which uses 90-day weather forecast data (5-km grid resolution) provided by the Australian Bureau of Meteorology.