Macquarie Island’s invertebrate fauna is made up of over 350 species, with 44 endemics and at least 40 species of established non-native invertebrates. Most established non-native invertebrates on Macquarie Island are small and flightless detritivores. In addition, there are several large-bodied, non-native macro-detritivores, including two predatory flatworm species (the focus on this report), Kontikia andersoni and Arthurdendyus vegrandis. Invertebrate monitoring was undertaken across Macquarie Island, with field surveys between 2015-2018. We found that Kontikia andersoni has significantly expanded its range since a previous survey in 2004. Arthurdendyus vegrandis was not detected during the surveys, despite additional focused searches in the area of its recorded occurrence in 2004.