Sea-level rise resulting from climate change is impacting coasts around the planet. There is strong scientific consensus about the amount of sea-level rise to 2050 (0.24-0.32 m) and a range of projections to 2100, which vary depending on the approach used and the mitigation measures taken to reduce carbon emissions. Despite this strong scientific consensus regarding the reality of climate change-related sea-level rise, and the associated need to engage publics in adaptation and mitigation efforts, there is a lack of empirical evidence regarding people's understanding of the issue. Here we investigate public understanding of the amount, rate and causes of sea-level rise. Data from a representative sample of New Zealand adults showed a suprising tendency for the public to overestimate the scientifically plausible amount of sea-level rise by 2100 and to identify melting sea ice as its primary causal mechanism. These findings will be valuable for scientists communicating about sea-level rise, communicators seeking to engage publics on the issue of sea-level rise, and media reporting on sea-level rise.