Fisheries management requires that stock assessments account for perturbations that alter the way fish populations respond to fishing pressure. Southern Rock Lobster (Jasus edwardsii) are targeted by regional fisheries of Australia’s southern continental shelf with an annual commercial catch of ~4000 tonnes valued at ~AUD $200 million. Estimates of size at sexual maturity (L50) are a key population parameter used in the management of Southern Rock Lobster (Jasus edwardsii) and provide a guide to setting minimum size limits and protecting egg production. However, estimates of L50 may vary spatially and temporally due to a range of factors that affect rock lobster growth, such as temperature or population density. In this study, we used data from stock assessment surveys between 1991 and 2015 to assess any changes in L50 in 5 regions of South Australia’s Southern Rock Lobster Fishery. Increases in L50 were evident since 1991 in 4 out of 5 regions assessed, and may warrant the refinement of minimum size limits used in the fishery. Possible causes for increases in L50 are discussed, with a particular focus on changes in sea surface temperature (SST) and population density over the time period.