Incidental bycatch in gill nets is the most serious of the global threats to marine mammals. Consequently, many management agencies wish to implement practical and efficient bycatch mitigation systems that both protect species of conservation concern and are readily adopted by fishermen. Australian snubfin Orcaella heinsohni and humpback dolphins Sousa chinensis occur in small fragmented populations along most of the remote coast of subtropical and tropical Australia, where they are caught in gillnet fisheries operated from small vessels. We experimentally investigated whether a commercially available acoustic alarm modified the behaviour of each of these species in the absence of a net. The movements and behaviour of both species changed subtly when the pingers were active, but the likelihood of the animals leaving an area was not significantly different from the controls. Our results suggest that this technological approach may not be effective in reducing the bycatch of these species. We suggest that further experimentation is unlikely to be cost effective and that government agencies should work with fishers and scientists to explore alternative mitigation measures.