In Australia, wheat is traditionally sown in a window from early–late autumn (April–May), to ensure flowering occurs at an optimal time in spring. This optimal flowering period (OPF) is defined early, by the risk of reproductive frost damage, and later, by high temperatures and terminal water stress during grain filling. Recent Australian research has demonstrated that slower developing spring or winter cultivars are capable of increased water limited yield potential across a range of environments. However, the longer phase duration of these cultivars require earlier sowing dates to meet OFP and to maximise yield potential. Here, we evaluated flowering and yield responses of a range of commercial cultivars and breeding lines that differed in their developmental rate and phenology genes in field experiments sown from early April–June at 12 locations from 2014–2018. We then employed these field data to optimise the parameters in a cropping systems model, APSIM, to simulate wheat yield and flowering date across the locations with historical climatic records (1961–2018) for defining the OFP for each location. We determined that the OFP, and the genotype x sowing date combinations which achieved the OFP and maximum yield varied across the locations, and within seasons; largely attributed to seasonal water supply and demand and temperature extremes. This affirms that cultivar selection has a critical impact on yield potential, and can be optimised by Australian growers by managing sowing time.