Marine biodiversity has declined over the past 50 years due to fishing, pollution, and climate change. To protect Victoria’s unique marine biodiversity, the state established no-take marine parks and sanctuaries in 2002. This study analyses 20 years of underwater visual census data to track changes in fish and invertebrate distributions over time. Using joint species distribution models, we assessed changes in species assemblages inside and outside of marine parks and across environmental gradients. Our findings indicate that 19 of the 56 fish species had a significant positive response to marine parks. Species richness was also notably higher inside marine parks compared to outside. Over the 20-year period, many fish species' distributions increased within marine parks and had decreased in areas open to fishing. However, five species, including Notralabrus fucicola and Acanthalutere viteger, declined across the study. The invertebrate model results were mixed, with Jasus edwardsii showing increased abundance inside marine parks, while Haliotis rubra decreased statewide. Other species of gastropod and starfish significantly declined. These results highlight the benefits of no-take Marine Parks in protecting marine biodiversity, but they also suggest that other environmental impacts (e.g., climate change) may be driving decreases in species, particularly invertebrates.