Recent advancement in compact, relatively low-cost, and highly manoeuvrable micro–Autonomous Underwater Vehicles (micro-AUVs) provide a alternate platform for collecting high-resolution measurements in the coastal ocean, where conventional methods often struggle to address fine-scale variability. In this study, a NemoSens micro-AUV by RTSys, equipped with a Conductivity-Temperature-Depth (CTD) sensor, was deployed in Perth coastal waters to investigate the fine-scale variability in hydrographic structure and plume dispersion. Field surveys were conducted at Coogee, where coastal waters were influenced by a river plume, and at Swanbourne, where the dispersion of a wastewater plume was monitored. The micro-AUV followed a sawtooth trajectory to obtain high-resolution temperature and salinity measurements, enabling the spatial extent, three-dimensional structure and mixing characteristics of coastal plumes to be resolved. At Coogee, the micro AUV captured a low-salinity surface layer, indicating river-influenced coastal water overlying the more saline ambient seawater. At Swanbourne, the surveys identified wastewater plume features, including low salinity and enhanced stratification near the outfall. Plume structure and mixing were further examined using density, potential energy anomaly, and salinity-based dilution estimates. The results demonstrated that micro AUVs are an effective tool for capturing small-scale variability in shallow coastal environments and highlight their broader potential for environmental monitoring.