A growing volume of evidence shows that the broad-scale biogeographic redistribution of species is occurring in response to increasing global temperatures. The present study documents poleward movements of up to eight species of nominally ‘tropical’ macroinvertebrates (molluscs, polychaetes, crustaceans and foraminifera) from intertidal mudflats on the south east coast of Australia. The speed of movement was comparable with that for Australian marine fauna generally, but was particularly fast for worms and molluscs (~70–300 km decade−1) and may be facilitated by the southward flowing East Australia Current. Further, two temperate taxa appear to have extended their ranges northwards. Changes in species biogeographic ranges raises questions surrounding the response of ecological processes within the altered and novel species combinations, including processes that underpin valuable ecosystem services. Using biological traits analysis to investigate how the observed species range changes might have impacted mudflat ecosystem functioning, and to predict the possible impacts of further poleward movements of tropical taxa. Our models suggest the changes to date, and those likely to occur in the near future, are within the envelope whereby ecological functioning is maintained by functional compensation and redundancy within the mudflat assemblage. However, in the most extreme scenario the replacement of temperate by tropical taxa resulted in major changes in ecological functioning with potential impacts on nutrient cycling and C-cycling, undermining the potential of these mudflats to continue to deliver critical ecosystem services. The widespread nature of biogeographic range shifts and the value of coastal systems should add further weight to calls for global action to mitigate global temperature change.