Biological invasions and alterations in natural flow regimes threaten native freshwater communities worldwide. Low flow conditions could interact with biological invasions to either exacerbate or offset their impacts. However the mechanisms driving any interacting effects of low flow and non-native species on native freshwater fish are poorly known, hindering management efforts. In Aotearoa - New Zealand, some native non-migratory galaxiid (NMG) populations co-occur with non-native salmonids, with water abstraction possibly playing an important role in controlling these species interactions, and thus their spatial distributions. I used field surveys to evaluate whether drying stream conditions benefitted NMGs at the expense of trout in Canterbury high country streams. Three electrofishing surveys were conducted (December – March) in streams containing NMG (Galaxias vulgaris and G. paucispondylus) in both perennial and drying reaches to assess abundance, body size and growth rate patterns between streams with high abundances (n=2), low abundances (n=2) or no brown trout (n=3; Salmo trutta). Flow effects on NMG were density-dependant on trout-density and varied depending on trout treatment. NMG abundance and biomass were significantly higher with trout absent compared to present at high abundance. NMG growth rate also decreased, or was flat, as NMG abundance increased when trout were absent, showing that growth rate was also density-dependant. However streams containing high trout abundance had significantly lower NMG abundance and biomass in perennially-flowing reaches, and moderate NMG abundances & biomass in drying reaches. NMG growth rate was also negatively affected by trout at high densities. By comparison, low density trout streams showed no clear differences in NMG abundance or biomass, between reaches. However, NMG growth rate was highest when trout were present at low densities, possibly due to facilitation by trout. Thus, although low-flow conditions reduce predatory effects of the non-natives, populations of native fish were also suppressed by low flow. To conserve native fish, managers should aim to: (1) restore and protect natural flow regimes to maintain suitable fish habitat, (2) actively manage harmful non-native species.