Hybridisation and introgression are common within Australian Freshwater fishes, not only under natural conditions but also as an emerging threat to their conservation. Multiple species of endemic rainbowfishes are facing extinction due to introgressive hybridisation with Melanotaenia splendida and in some cases the cause is unclear. To better understand this process and its drivers I have been investigating the mate preferences, reproductive compatibility, thermal performance, and genetic patterns of M. sp. 'Malanda', M. splendida and their hybrids. This talk will provide a preliminary look at the work investigating the processes driving species replacement through hybridisation in Australian rainbowfishes.