Across the globe, native species are being outcompeted and often reach extinction due to introduced species becoming invasive. Previously confined to their native areas due to geographical, ecological, or environmental barriers that have prevented them from expanding, human impacts have resulted in a significant increase in the number of introduced species. Such species become invasive when they begin to expand their range demographically and typically cause negative impacts in the new environment. There is no current model that allows us to predict and prevent future biological invasions, though next generation sequencing, population genomics analysis, and experimental laboratory manipulations are helping to fill critical gaps in our understanding of the invasion process. My first analysis (Chapter 2) explored the ability of population genetics analyses of single nucleotide polymorphism (SNP) data to identify hybridisation levels and the rate of admixture occurring in wild populations of Calliphora hilli and Calliphora stygia – two invasive blowflies found in New Zealand and originally from Australia. I analysed samples from various locations and found patterns of population genetic connectivity and structure that supported Australia as the source of the New Zealand invasion for both species. This research provided highly valuable new insights into the population structure of these two species, with hybridisation and gene flow playing a key role in their respective biological invasions. My second analysis (Chapter 3) first explored the population structure of the highly invasive blowfly species, Callipohra vicina, using SNP data to analyse population genomic patterns, such as genetic diversity and admixture. Following this, low genetic diversity colonies were generated from isofemale lines to simulate an invasive population that had undergone a genetic bottleneck. These low diversity lines were compared to relatively high diversity lines for a number of traits, including fecundity, body size, developmental rate, and lifespan to determine the effects of genetic diversity on population fitness. We found genetic differentiation between North and South Island New Zealand populations in the wild, while high diversity lines outcompeted low diversity lines for all measured traits in the laboratory. These results demonstrated the importance of genetic bottlenecks on invasion scenarios and suggested interesting new ideas for follow-up research. Predicting and preventing future invasions is a significant current gap in invasion biology. Population genomic and ecological assays can together help to fill this gap to help us identify the mechanisms underlying invasive success.