Average yield of canola in the high-rainfall zone (HRZ) of southern Australia are about half the predicted potential yield based on seasonal water supply. Current cultivars of canola that are available to growers were not bred specifically for the HRZ and tend to be short-season types aimed at escaping water stress during grain filling in the drier regions of the cropping belt. In the HRZ, these cultivars fail to utilise all available growing-season water due to early maturity. Field experimentation and crop simulation studies across the HRZ landscape of south-eastern Australia were used to determine the increased yield potential of longer-season canola cultivars compared with short-season cultivars. In this study the Catchment Analysis Tool spatial modelling framework was used to determine the expected canola yields of three cultivars across the entire HRZ of south-eastern Australia. Hyola50 (‘spring-short’) was used to represent the current recommended spring-type canola cultivar within the HRZ and was evaluated against an unreleased long-season spring-type cultivar CBI8802 (‘spring-long’) and a newly released winter-type cultivar Taurus (‘winter’). Spring-long outperformed spring-short across much of the study area. Yield advantages of winter over spring-short were mainly confined to the coastal fringe of Victoria and Tasmania and small pockets in New South Wales where at one location the average yield over 50 growing seasons exceeded spring-short by up to 60% or 1.4 t/ha. The superior performance of spring-long, (up to 17% or 0.9 t/ha at one location) was over a wider area than winter (26.4 compared with 8.8 million ha for winter) and although the magnitude of the yield increase over spring-short was not as great as winter at some locations, the overall result determined that spring-long had the greater production potential. The superior performance of spring-long beyond the HRZ challenges the trend of selecting earlier maturing cultivars by current breeders following the abnormal sequence of dry years in an attempt to minimise yield loss due to water stress during grain filling. This study has provided breeders, growers and advisors with information on where in the HRZ a longer-season canola cultivar can be grown to improve overall crop productivity. It has also provided evidence that new canola types may be required to maximise grain yields not only for the HRZ but potentially also in lower rainfall regions.