A plant’s mating system influences its success in invading new environments. Self-compatible species benefit from reproductive assurance during the early phases of the invasion process when founding populations are small, but may suffer the fitness costs associated with selfing. In contrast, self-incompatible (SI) plants often suffer mate limitation in small populations but maintain progeny fitness. SI plants often have complex dominance interactions among S-alleles that increase mate availability by allowing mating among half- and full-sib relatives. Such biparental inbreeding generally has smaller fitness effects than selfing. In this study we assessed if promotion of biparental inbreeding through selection for intermediate dominance S-alleles is a ecologically and evolutionarily viable strategy for SI invaders that simultaneously maximises mate availability and progeny fitness. Specifically, we explored the tradeoff between mate availability and progeny fitness in five Australian populations of the SI weed wild radish (Raphanus raphanistrum). To do this we used a two-generation controlled cross experiment to generate self, full-sib, half-sib and unrelated individuals and measured their life-time fitness under field conditions. Diallel crosses were conducted to estimate S allele numbers, frequencies of dominance interactions and mate limitation in each population. We found large negative effects of selfing on fitness in all populations across a broad range of vegetative growth and plant reproductive characters e.g biomass, flowering, seed set. Biparental inbreeding consistently resulted in smaller but significant negative fitness effects. Diallel crosses showed dominance among S alleles contributed to increased mate availability through facilitation of mating among relatives, though the degree of this effect varied among populations. Interestingly the importance of dominance in freeing up mate availability in a population was positively correlated with the observed severity of inbreeding depression expressed in selfed individuals. Taken together these data suggest that dominance among S alleles may play an important role in freeing up mate availability in small populations of wild radish. As biparental inbreeding was demonstrated to be less severe in its effects on fitness than selfing, selection for intermediate dominance alleles may provide a novel ecological strategy to maintain population viability during the colonisation process, while preserving self-incompatibility.