<p dir="ltr">The global increase in frequency and severity of marine heatwaves (MHWs) is highlighting the impact these extreme climatic events can have on marine ecosystems. Throughout the summer of 2021/2022 northeastern Aotearoa/New Zealand suffered an unprecedented MHW. Worst impacted areas, such as the semi-enclosed Te Moananui-ā-Toi/Hauraki Gulf, experienced more than three continual months of temperatures at or above the climatological mean maximum (20.7°C). During this period, we observed a rapid increase in the abundance and cover of the invasive, warm-affinity colonial ascidian <i>Symplegma brakenhielmi</i> on temperate rocky reef<i>. </i>Population expansion of this species has not previously been linked to MHW events. Benthic monitoring as water temperatures cooled showed a sharp decrease in <i>S. brakenhielmi</i> abundance, but not complete disappearance, and the coverage of individually monitored colonies also declined. There was no observed increase in abundance or cover throughout the summer of 2022/2023, a potential consequence of cooler water temperatures and multiple cyclones. Observed impacts included the growth of <i>S. brakenhielmi</i> over other sessile invertebrate and macroalgal species, as well as on highly mobile spiny lobster, suggesting that this species can have a variety of impacts on temperate rocky reefs. These findings highlight how MHW can facilitate the rapid expansion and integration of non-native, warm-affinity species into temperate reef ecosystems and provide insight into what we can expect in the future as ocean temperatures continue to warm.</p>
