Extreme marine events can generate cascading ecological and social consequences, yet the human wellbeing dimension of these events including harmful algal blooms (HABs) remains under‑recognised within HAB science and management. During South Australia’s unprecedented 2025 HAB, we examined psychological responses among residents (N = 628), providing the first empirical evidence of how a large‑scale marine ecological crisis can disrupt mental health and human–nature relationships. Quantitative findings revealed marked eco‑anxiety, with over 60% of participants reporting rumination (69%) and personal inefficacy (63%), alongside elevated affective symptoms (54%) and behavioural disruptions (20%) in the two weeks prior to survey completion. Eco‑anxiety was positively associated with ocean visitation frequency (rs = .102–.266) and significantly higher among women across all subscales (p ≤ .028). Qualitative analysis identified themes of ocean disconnection, emotional distress, and reduced blue‑space engagement, illustrating how HABs can erode restorative coastal relationships.