The need for cooling or heating of residential and commercial properties directly impacts electricity demand, greenhouse gas emissions, local climate and local and regional air quality. In summer energy demand for air conditioning leads to air and greenhouse gas emissions from centralised fossil-fuel power stations and places strains on the reliability of electricity supply in the National Electricity Market. During cooler weather, heating methods such as residential wood heaters add significantly to local and regional air pollution, while gas and electrical heating lead to greenhouse gas emissions. The demand for heating or cooling energy can be inferred using measures of heating degree days (HDD) and cooling degree days (CDD). We used Bureau of Meteorology long term climate data (ACORN-SAT) to investigate the trends in HDD and CDD for major population centres in Australia. We found that all capital cities (except Darwin) have seen statically significant (α=0.95) reductions in HDD over the past century. Similarly, all cities have seen significant (α=0.90) increases in CDD over the same period. Comparing coastal population centres with inland centres shows even stronger reductions/increases in HDD/CDD. This implies that climate changes alone should have driven lower energy demand for space heating across Australia, but increased energy demand for cooling (notwithstanding socio-economic drivers of electricity demand). Projected changes in HDD/CDD from a regional climate model (NARCliM) were also assessed to investigate likely changes in the future and show the observed trends continuing throughout the 21st century. The results support that both observed and projected changes in climate should be considered by policy makers when considering urban heating and cooling demand. This is important when considering building design, urban planning, and the design of integrated energy systems.