Advances in satellite ocean colour remote sensing have enabled systematic detection of episodic phytoplankton blooms — termed High Chlorophyll Events (HCEs) — revealing their growing threat to marine infrastructure and recreational amenity across urban coastal environments globally. Physical and biological drivers controlling sporadic HCEs in oligotrophic systems, where warm boundary currents typically suppress productivity, remain poorly understood, creating critical gaps in predictive capacity for water security management and recreational closures. This study examines HCEs along Perth's southwest coast, Western Australia, where HCEs threaten seawater desalination operations and Trichodesmium cyanobacteria HCEs disrupt recreation. Both occurring despite nutrient-poor conditions sustained by the Leeuwin Current. Daily chlorophyll-a concentrations (2018–2025) were derived from Sentinel-3 products at 300 m resolution for Cockburn Sound and Whitfords Lagoon. HCEs were classified using a dual-threshold percentile-based framework, distinguishing extreme events (eHCEs) from anomalous events (aHCEs). Both sites exhibited bimodal seasonality with environmental drivers: Cockburn Sound HCEs were governed by Swan River discharge and storm-regenerated nutrients, whilst Whitford Lagoon HCEs were controlled by hydrodynamic retention near Lal Bank under low-energy conditions and dissipated by storm-induced flushing. These site-specific mechanisms underpin morphology-resolved predictive models for operational coastal management, safeguarding critical water infrastructure and recreational resources under intensifying climate variability.