The pearl cichlid, Geophagus brasiliensis is an aggressive aquarium fish from Brazil that has invaded urban rivers across the world and poses a threat to native fish. Instream barriers in urban rivers can slow or halt the spread of alien fish. The Canning River is a 110 km long river in the urban metropolis of Perth (Western Australia) and contains the cichlid. Netting surveys indicate that the cichlid is extending its range upstream but remains below a weir midway along the river. However, before managers attempt to use the weir to prevent the species upstream spread there is a need to firstly confirm that the cichlid has not crossed the barrier. The low abundance of invading species and the imperfect detection of fish by nets means that uncertainty exists about its distribution. This study used three monitoring techniques (active eDNA, passive eDNA and fyke netting) and occupancy modelling to assess changes in the species’ distribution over a 6-year period (2015 to 2021). We also sought to reveal the number of replicates needed of each monitoring technique to provide a 90% probability of detecting the species at a site where it is present. We found that active eDNA sampling was the best at detecting G. brasiliensis, followed by fyke netting and passive eDNA sampling. We also found that the weir has not prevented the upstream spread of G. brasiliensis. We provide information on the number of replicates needed of each gear to detect this species in the study system. Our results contribute to understanding the distribution of G. brasiliensis in the Canning River and the application of passive eDNA sampling in freshwater systems.