It is likely that transgenic canola expressing genes encoding resistance to glyphosate and glufosinate ammonium will be introduced into the Australian cropping system in the next few years. One risk associated with the introduction of such cultivars is the release of herbicide resistance genes into weedy relatives of canola. This review examines the currently available experimental information regarding the possibility of gene flow from canola to weedy relatives. Three species are identified as having the potential to outcross with canola, Brassica juncea, B. rapa, and Raphanus raphanistrum. Two of these species are not yet widespread weeds of the southern Australian cropping zone. In contrast, R. raphanistrum is already a major weed in Australia with existing resistance to ALS-inhibiting herbicides. Information is urgently needed to determine whether successful hybrids between B. napus and R. raphanistrum can be produced under Australian conditions. Major deficiencies in the existing information are identified in relation to some other important weed species within the southern Australian cropping zone. Further studies are required to determine the out-crossing potential of canola to B. tournefortii, Diplotaxis tenuifolia, Sisymbrium officinale, and S. orientale if transgenic canola is to be safely and responsibly introduced into Australia.