The responses to water stress during the post-flowering period of two mustard breeding lines (8126.96.36.199 and Muscon) and a commercial canola cv. Monty were tested in the field at Merredin in the low-rainfall Mediterranean-type environment of Western Australia. Three water-stress treatments were imposed using supplemental irrigation and a rain-exclusion shelter. Increasing water stress in the post-flowering period significantly reduced dry matter production and seed yields. Harvest index was slightly increased by mild stress, but reduced back to control levels by severe stress. Pods/plant, seeds/pod, and 1000-seed weight were all reduced by water stress. Dry matter production was higher in mustard than in canola, due to its greater water use and radiation interception. Water-use efficiency (WUE) for dry matter production and radiation-use efficiency (RUE) were higher in mustard than in canola. WUE for dry matter production and RUE were insensitive to the levels of water stress in mustard in this experiment, but declined significantly in canola. The greater water use in mustard and insensitivity of WUE for dry matter production and RUE to water stress were attributed to significantly higher levels of osmotic adjustment in mustard, although osmotic adjustment was also observed in canola.Despite this, canola seed yield was not significantly lower than the seed yield of the better mustard genotype, although stress caused a significantly greater percentage yield reduction in canola. This is because canola had a higher harvest index, which also meant it had higher WUE than mustard for grain production under mild stress. Mustard’s poorer harvest index was due to more of the dry matter being invested in stem and, in the case of cv. Muscon, to a short reproductive duration and a low proportion of pod weight allocated to seed.Canola had significantly higher seed oil concentration than mustard, which meant that it produced higher total oil yield despite sometimes producing lower seed yield. However, its oil concentration was reduced more by stress than mustard’s, so under the most severe stress conditions, both mustard genotypes produced higher total oil yield. Mustard has potential as an oil-producing crop in the low-rainfall Mediterranean-type environments of Western Australia, but improved genotypes, greater harvest index, and greater seed yield are required.