Urbanisation is placing a greater stress on the natural environment. Urban catchments have high coverage of impervious surfaces with modified stream hydrology and water chemistry. In urban areas it is well documented that opportunistic invasive species colonise and displace many native species once habitat is degraded, and urban stream are no exception. The freshwater gastropods Physa acuta (Physidae) and Potamopyrgus antipodarum (Hydrobiidae) are invasive species (from Europe and New Zealand) and are have become widespread throughout the waterways of south-east Australia. The aim of this study was to investigate the distribution of the pollution tolerant gastropod species P. acuta and P. antipodarum and the pollution sensitive mayfly family (Leptophlebiidae) in streams with varying degrees of urban development. Research was undertaken on the Georges River catchment which has streams with undisturbed to highly urbanised sub-catchments. Quantitative sampling of Leptophlebiidae, Physidae and Hydrobiidae was undertaken from 17 sites across the Georges River catchment. We found Physa acuta and Potamopyrgus antipodarum were absent from non-urban catchments, whereas Leptophlebiidae nymphs were abundant. These streams had water quality with low pH (<6) and low salinity (<250 µS/cm). In contrast, Leptophlebiidae were absent from highly urbanised catchments. Water chemistry at the highly urban stream catchments had higher pH (>7) and higher salinity (>400 µS/cm). Additionally urban streams had high nutrient levels in comparison to reference streams. Results of this study indicate that modification to fresh water chemistry due to urbanisation is associated with increased invasive gastropod density and to the decline of pollution sensitive mayfly. The invasive snails were present in some peri-urban streams and common in urban streams. A puzzling finding was that invasive gastropods have an apparent intolerance of the water chemistry of the cleanest non-urban streams. We suspect that the naturally low pH may be protecting these streams from colonisation by the invasive snails. We question whether the ANZECC pH guideline (minimum pH of 6.5) is prudent for protecting freshwater ecosystems from invasive gastropod species.