The East Australian Current (EAC) is a highly dynamic western boundary current (WBC) that exerts significantly influences on the marine ecosystem off the southeast margin of Australia. In this study, for the first time a quantitative mapping of the EAC system was conducted using Himawari-8 Sea Surface Temperature (SST) data and a Topographic Position Index (TPI) based image processing technique. The validation using the Bluelink ReANalysis data suggested good reliability of the mapping results.  Subsequent quantitative examination of the EAC encroachment provided new insights into the EAC’s cross-shelf movement which is a major driving factor of the adjacent slope and shelf circulation. Off the coast of New South Wales, large-scale and high-frequency EAC encroachment was observed, being every 65-80 days upstream (28-32.25 ºS) and every 90-100 days downstream (32.25-37.30 ºS), which is associated with the EAC’s periodic eddy shedding. Particularly, the eddy variability likely has a greater impact on the downstream, as the EAC’s displacement exhibits a ~20 days longer period and a double amplitude in this region. Higher-frequency (sub-monthly) and smaller-magnitude EAC encroachment was also observed which is possibly associated with EAC meanders and frontal eddies. In upstream, the EAC encroachment also exhibits seasonality, being 10-20 km closer to the coast in austral summer, which is likely due to the seasonal EAC broadening. In the extension zone (37.30-44 ºS), we observed maximum EAC encroachment in summer, which is an expression of the seasonal boundary flow off the east coast of Tasmania. A further study using 26 years of AVHRR SST data has confirmed the seasonality of the EAC encroachment in upstream area, which provides new insights into the seasonal coastal upwelling and shelf circulation.