Estuaries are becoming increasingly threatened by human activities and their management must consider impacts of multiple human threats. Cumulative impact maps are a popular tool to identify the distribution and intensity of threats in estuaries. Few studies, however, have demonstrated strong correlations between cumulative impacts and ecosystem condition, questioning their use for management. Here, we develop a cumulative impact map for Posidonia australis in Pittwater, NSW, Australia, using spatial data on threats to seagrasses (water quality, boat moorings, jetties and vessel traffic). We then test how well cumulative threat scores explained changes in Posidonia australis cover between 2005 and 2019. Clear gradients in cumulative threats to Posidonia australis were observed in Pittwater. Unsurprisingly, the greatest intensity of threats occurred along the highly urbanised eastern shores and upper reaches of the estuary. Using a generalised additive model, contemporary change in Posidonia australis cover was negatively correlated to cumulative threat scores. Between 2005 and 2019, Posidonia australis cover increased in areas where cumulative threats were low, while declines were observed in areas where cumulative threats were high. Our results suggest management actions aimed at reducing or limiting cumulative threats to low and moderate levels may help protect Posidonia australis in Pittwater.