The North American calanoid copepod Skistodiaptomus pallidus is an emerging invader globally, with non-indigenous populations recorded from constructed waters in New Zealand, Germany and Mexico since 2000. We examined the effects of S. pallidus establishment on the zooplankton community of a natural lake, Lake Kereta, where it was first recorded in late-2008, coincident with releases of domestically cultured grass carp (Ctenopharyngodon idella). Although not present in any of our samples prior to August 2008, S. pallidus was found in all samples collected in the subsequent five years. ANOSIM indicated zooplankton community composition significantly differed between samples collected before and after S. pallidus invasion, whether the invader was included in the analysis or not. Zooplankton species affected most greatly were the copepods Calamoecia lucasi and Mesocyclops sp., which decreased in their relative importance, and the cladocerans Bosmina meridionalis and Daphnia galeata, which increased. Rotifer species were relatively unaffected. As the length of grass carp released were >6.5 cm, direct predatory effects by this species on the zooplankton community are unlikely. Associated reductions in macrophyte biomass could explain increases in the relative abundances of planktonic cladocerans (B. meridionalis and D. galeata). However, the effect of macrophyte reduction by grass carp on zooplankton communities is considered to be limited elsewhere, while the reduced macrophyte biomass cannot explain the decrease in relative abundance of the native planktonic calanoid copepod C. lucasi. Competition between C. lucasi and S. pallidus is the most compelling explanation for the reduction in importance of the native calanoid copepod species. Skistodiaptomus pallidus appears to have undergone a “boom-and-bust” cycle in Lake Kereta, increasing in relative abundance in the first three years following establishment, before declining in importance.