© CSIRO 2016. Invasive species can have significant impacts on the diversity and productivity of recipient ecological communities. The kelp Undaria pinnatifida (Harvey) Suringar is one of the world's most successful invasive species but, although its purported impacts are strong, there is little empirical evidence that it displaces native species. Furthermore, as this species naturalises in local communities, its potential effects on community dynamics have not been well tested. Here, we test the ecological impacts of Undaria in intertidal communities in southern New Zealand using a combination of surveys, a 2.5-year press-removal experiment and in situ measures of net primary production to gauge its impact on community structure and productivity. Undaria had transient effects on the composition of communities, affecting two seasonally abundant species in 1 year, but these impacts did not persist into the following year. Overall, there were only small effects of Undaria removal on diversity and abundance of native algae and invertebrates at two sites. However, the presence of Undaria more than doubled net primary production of recipient communities during its annual peak abundance when it increased biomass by 606gDWm-2. We conclude that the invasion of Undaria represents an additional and substantial carbon subsidy to coastal ecosystems with potentially positive effects on nearshore productivity.