Genotype by environment interactions affecting post-weaning body weight in Australian sheep were investigated using a linear reaction norm incorporating genomic information. The definition of the environmental covariable used in the reaction norm was the best linear unbiased estimation of contemporary group effects for post-weaning growth rate. Significant variation in slope was estimated, and genetic correlations between low, medium and high growth environments ranged from 0.61 to 0.94, suggesting the presence of genotype by environment interaction. A putative QTL was detected on chromosome 11, significantly associated with both the intercept and slope of the reaction norm. Overall, SNP effects for the intercept and slope were highly correlated (0.87). The results suggest that selection based on (genomic) breeding values for the intercept and slope could yield animals that are more robust.