Native to the northern hemisphere, introduced trout species are now well established throughout the cool waters of the southern hemisphere. It is widely recognised that their introduction has led to localised extinction, significant range reductions and serious conservation concern for many native small bodied fishes, especially Galaxiidae. Today many Galaxiids are considered highly threatened and exist only in headwater streams, protected from trout invasion by downstream barriers such as waterfalls. Despite the reported negative effects of trout on Galaxiids demonstrated by their largely contrasting current distributions, data-based records of these effects, interactions and subsequent Galaxiid decline are scarce. Poorly documented fish communities prior to trout introduction and the rapid dispersal of trout before thorough survey work are the likely reason for infrequent records, rather than a lack of negative trout effect. Better understanding of the location of existing trout barriers will benefit Galaxiid research and conservation efforts, including determining the current range of both trout and Galaxiids, and locating new streams where additional Galaxiid populations can be established. This study documents the reduction of the range of trout in Lees Creek in the Cotter River catchment in the Australian Capital Territory, and the subsequent expansion of native Galaxias olidus in the now trout-free reach of this creek. Historic decline of G. olidus was not recorded when trout were introduced but this study details an observed improvement of G. olidus range and simultaneous contraction of trout range. Digital elevation models were used to identify likely barriers along the creek, and fish surveys were undertaken to determine the extent of range of both introduced trout and native G. olidus. Findings revealed several likely barriers along Lees Creek, and showed trout distribution has reduced by approximately 5 km since the late 1990s. These findings have important implications for the conservation of threatened small bodied fish elsewhere, and highlight the need to understand where in-stream barriers are located within a stream network, even those downstream or within the current range of introduced trout.