From midnight of 26 March 2020, New Zealand became one of the first countries to enter a strict lockdown to combat the spread of COVID-19. The lockdown banned all non-essential services and travel both on land and sea. Overnight, the country's busiest coastal waterway, the Hauraki Gulf Marine Park, became devoid of almost all recreational and non-essential commercial vessels. An almost instant change in the marine soundscape ensued, with ambient sound levels in busy channels dropping nearly 3-fold the first 12 hours. This sudden drop led fish and dolphins to experience an immediate increase in their communication ranges by up to an estimated 65%. Very low vessel activity during lockdown (indicated by the presence of vessel noise over the day) revealed new insights into cumulative noise effects from vessels on auditory masking. For example, at sites nearer Auckland City, communication ranges increased approximately 18 m (22 %) or 50 m (11 %) for every 10% decrease in vessel activity for fish and dolphins, respectively. However, further from the city and in deeper water these communication ranges were increased by approximately 13 m (31 %) or 510 m (20 %). These new data demonstrate how noise from small vessels can impact underwater soundscapes and how marine animals will have to adapt to ever-growing noise pollution.