Abstract: The Indonesian-Australian Basin, located between Indonesia and Australia, is influenced by both the Indian Ocean circulation and the Indonesian Throughflow (ITF), resulting in complex water-mass properties, types, and distributions. Based on CTD observations collected during the 2020 northwest monsoon season, this study analyzes the vertical sections and T-S diagrams of temperature, salinity, and dissolved oxygen concentration. The results show that the 0-600 m layer is strongly influenced by the ITF and the Eastern Gyral Current (EGC), with pronounced meridional differences in water properties, whereas horizontal differences are relatively weak below 600 m and vertical structures become more prominent. The K-means++ clustering algorithm was subsequently applied in the vertical and horizontal dimensions to classify the 0-1 500 m water column into 10 water masses, and the depth ranges and hydrographic characteristics of each water mass were systematically analyzed. Principal component analysis was then conducted for the horizontally clustered groups. The results indicate that the relative contributions of temperature, salinity, and dissolved oxygen concentration differ among water masses, further supporting the validity of the horizontal clustering results. The clustering results based on contemporaneous Argo data are generally consistent with the CTD-based classification for surface and subsurface water masses; however, the discrimination of deeper water masses is relatively weak because Argo data lack dissolved oxygen concentration observations. Compared with previous studies, this work separately identifies a high-oxygen water mass in the upper-intermediate water layer, provides a more detailed water-mass classification, and supplements the depth ranges and hydrographic characteristics of the identified water masses. Circulation analysis indicates that the ITF and EGC contribute to water-mass transport, while eddies also exert an influence. Low-salinity, low-oxygen water masses originate mainly from the Indonesian seas, high-salinity water masses primarily originate from the southern Indian Ocean, and some water masses originate from the northern Indian Ocean.